Difference between revisions of "2019 AMC 8 Problems/Problem 16"
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− | ==Problem | + | ==Problem== |
Qiang drives <math>15</math> miles at an average speed of <math>30</math> miles per hour. How many additional miles will he have to drive at <math>55</math> miles per hour to average <math>50</math> miles per hour for the entire trip? | Qiang drives <math>15</math> miles at an average speed of <math>30</math> miles per hour. How many additional miles will he have to drive at <math>55</math> miles per hour to average <math>50</math> miles per hour for the entire trip? | ||
<math>\textbf{(A) }45\qquad\textbf{(B) }62\qquad\textbf{(C) }90\qquad\textbf{(D) }110\qquad\textbf{(E) }135</math> | <math>\textbf{(A) }45\qquad\textbf{(B) }62\qquad\textbf{(C) }90\qquad\textbf{(D) }110\qquad\textbf{(E) }135</math> | ||
− | ==Solution 1(answer | + | ==Solution 1 (by looking at the answer choices)== |
− | The only option that is easily divisible by <math>55</math> is <math>110</math> | + | The only option that is easily divisible by <math>55</math> is <math>110</math>, which gives 2 hours of travel. And, the formula is <math>\frac{15}{30} + \frac{110}{55} = \frac{5}{2}</math>. |
− | And <math>\text{Average Speed}</math> = <math>\frac{Total Distance}{Total Time}</math> | + | And, <math>\text{Average Speed}</math> = <math>\frac{\text{Total Distance}}{\text{Total Time}}</math>. |
− | Thus <math>\frac{125}{50} = \frac{5}{2}</math> | + | Thus, <math>\frac{125}{50} = \frac{5}{2}</math>. |
Both are equal and thus our answer is <math>\boxed{\textbf{(D)}\ 110}.</math> | Both are equal and thus our answer is <math>\boxed{\textbf{(D)}\ 110}.</math> | ||
− | ~ | + | ==Solution 2== |
+ | To calculate the average speed, simply evaluate the total distance over the total time. Let the number of additional miles he has to drive be <math>x.</math> Therefore, the total distance is <math>15+x</math> and the total time (in hours) is <cmath>\frac{15}{30}+\frac{x}{55}=\frac{1}{2}+\frac{x}{55}.</cmath> We can set up the following equation: <cmath>\frac{15+x}{\frac{1}{2}+\frac{x}{55}}=50.</cmath> Simplifying the equation, we get <cmath>15+x=25+\frac{10x}{11}.</cmath> Solving the equation yields <math>x=110,</math> so our answer is <math>\boxed{\textbf{(D)}\ 110}</math>. | ||
+ | |||
+ | ==Solution 3== | ||
+ | If he travels <math>15</math> miles at a speed of <math>30</math> miles per hour, he travels for 30 min. Average rate is total distance over total time so <math>(15+d)/(0.5 + t) = 50</math>, where d is the distance left to travel and t is the time to travel that distance. Solve for <math>d</math> to get <math>d = 10+50t</math>. You also know that he has to travel <math>55</math> miles per hour for some time, so <math>d=55t</math>. Plug that in for d to get <math>55t = 10+50t</math> and <math>t=2</math> and since <math>d=55t</math>, <math>d = 2\cdot55 =110</math>, the answer is <math>\boxed{\textbf{(D)}\ 110}</math>. | ||
+ | |||
+ | ==Solution 4== | ||
+ | |||
+ | Let <math>h</math> be the amount of hours Qiang drives after his first 15 miles. Average speed, which we know is <math>50</math> mph, means total distance over total time. For 15 miles at 30 mph, the time taken is <math>\frac{1}{2}</math> hour, so the total time for this trip would be <math>\frac{1}{2} + h</math> hours. For the total distance, 15 miles are traveled in the first part and <math>55h</math> miles in the second. This gives the following equation: | ||
+ | |||
+ | |||
+ | <cmath>\dfrac{15+55h}{\frac{1}{2}+h} = 50.</cmath> | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | Cross multiplying, we get that <math>15 + 55h = 50h + 25</math>, and simple algebra gives <math>h=2</math>. In 2 hours traveling at 55 mph, the distance traveled is <math>\frac{2 \hspace{0.05 in} \text{hours}}{1} \cdot \frac{55 \hspace{0.05 in} \text{miles}}{1 \hspace{0.05 in} \text{hour}} = 2 \cdot 55 \hspace{0.05 in} \text{miles} = 110 \hspace{0.05 in} \text{miles}</math>, which is choice <math>\boxed{\textbf{(D)}\ 110}</math>. | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | ~TaeKim | ||
+ | |||
+ | ==Video Solution== | ||
+ | |||
+ | ==Video Solution by Math-X (First fully understand the problem!!!)== | ||
+ | https://youtu.be/IgpayYB48C4?si=8YldGqXbPzZfeA-z&t=4756 | ||
+ | |||
+ | ~Math-X | ||
+ | |||
+ | |||
+ | https://www.youtube.com/watch?v=OC1KdFeZFeE | ||
+ | |||
+ | Associated Video | ||
+ | |||
+ | https://youtu.be/5K1AgeZ8rUQ | ||
+ | |||
+ | - happytwin | ||
+ | |||
+ | https://www.youtube.com/watch?v=0rcDe2bDRug ~David | ||
+ | |||
+ | == Video Solution == | ||
+ | |||
+ | Solution detailing how to solve the problem: | ||
+ | |||
+ | https://www.youtube.com/watch?v=sEZ0sM-d1FA&list=PLbhMrFqoXXwmwbk2CWeYOYPRbGtmdPUhL&index=17 | ||
+ | |||
+ | ==Video Solution== | ||
+ | https://youtu.be/aFsC5awOWBk | ||
+ | |||
+ | - Soo, DRMS, NM | ||
+ | |||
+ | ==Video Solution== | ||
+ | https://youtu.be/btmFN_C1zSg | ||
− | + | ~savannahsolver | |
− | |||
− | + | ==Video Solution (The Fastest Way)== | |
+ | https://www.youtube.com/watch?v=IOhkO3c3c2A&ab_channel=SaxStreak002 | ||
+ | ~SaxStreak | ||
+ | ==Video Solution (MOST EFFICIENT+ CREATIVE THINKING!!!(BTW WE LOVE THIS ONE))== | ||
+ | https://youtu.be/JiTos1fFtUA | ||
− | + | ~Education, the Study of Everything | |
− | |||
+ | ==Video Solution by The Power of Logic(1 to 25 Full Solution)== | ||
+ | https://youtu.be/Xm4ZGND9WoY | ||
− | + | ~Hayabusa1 | |
==See Also== | ==See Also== |
Latest revision as of 09:32, 9 November 2024
Contents
[hide]- 1 Problem
- 2 Solution 1 (by looking at the answer choices)
- 3 Solution 2
- 4 Solution 3
- 5 Solution 4
- 6 Video Solution
- 7 Video Solution by Math-X (First fully understand the problem!!!)
- 8 Video Solution
- 9 Video Solution
- 10 Video Solution
- 11 Video Solution (The Fastest Way)
- 12 Video Solution (MOST EFFICIENT+ CREATIVE THINKING!!!(BTW WE LOVE THIS ONE))
- 13 Video Solution by The Power of Logic(1 to 25 Full Solution)
- 14 See Also
Problem
Qiang drives miles at an average speed of miles per hour. How many additional miles will he have to drive at miles per hour to average miles per hour for the entire trip?
Solution 1 (by looking at the answer choices)
The only option that is easily divisible by is , which gives 2 hours of travel. And, the formula is .
And, = .
Thus, .
Both are equal and thus our answer is
Solution 2
To calculate the average speed, simply evaluate the total distance over the total time. Let the number of additional miles he has to drive be Therefore, the total distance is and the total time (in hours) is We can set up the following equation: Simplifying the equation, we get Solving the equation yields so our answer is .
Solution 3
If he travels miles at a speed of miles per hour, he travels for 30 min. Average rate is total distance over total time so , where d is the distance left to travel and t is the time to travel that distance. Solve for to get . You also know that he has to travel miles per hour for some time, so . Plug that in for d to get and and since , , the answer is .
Solution 4
Let be the amount of hours Qiang drives after his first 15 miles. Average speed, which we know is mph, means total distance over total time. For 15 miles at 30 mph, the time taken is hour, so the total time for this trip would be hours. For the total distance, 15 miles are traveled in the first part and miles in the second. This gives the following equation:
Cross multiplying, we get that , and simple algebra gives . In 2 hours traveling at 55 mph, the distance traveled is , which is choice .
~TaeKim
Video Solution
Video Solution by Math-X (First fully understand the problem!!!)
https://youtu.be/IgpayYB48C4?si=8YldGqXbPzZfeA-z&t=4756
~Math-X
https://www.youtube.com/watch?v=OC1KdFeZFeE
Associated Video
- happytwin
https://www.youtube.com/watch?v=0rcDe2bDRug ~David
Video Solution
Solution detailing how to solve the problem:
https://www.youtube.com/watch?v=sEZ0sM-d1FA&list=PLbhMrFqoXXwmwbk2CWeYOYPRbGtmdPUhL&index=17
Video Solution
- Soo, DRMS, NM
Video Solution
~savannahsolver
Video Solution (The Fastest Way)
https://www.youtube.com/watch?v=IOhkO3c3c2A&ab_channel=SaxStreak002
~SaxStreak
Video Solution (MOST EFFICIENT+ CREATIVE THINKING!!!(BTW WE LOVE THIS ONE))
~Education, the Study of Everything
Video Solution by The Power of Logic(1 to 25 Full Solution)
~Hayabusa1
See Also
2019 AMC 8 (Problems • Answer Key • Resources) | ||
Preceded by Problem 15 |
Followed by Problem 17 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AJHSME/AMC 8 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.