Difference between revisions of "2011 AMC 12A Problems/Problem 12"
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=== Solution 1 === | === Solution 1 === | ||
− | WLOG let the speed of the river be 0. This is allowed because the problem never states that the speed of the current has to | + | WLOG let the speed of the river be 0. This is allowed because the problem never states that the speed of the current has to have a magnitude greater than 0. In this case, when the powerboat travels from <math>A</math> to <math>B</math>, the raft remains at <math>A</math>. Thus the trip from <math>A</math> to <math>B</math> takes the same time as the trip from <math>B</math> to the raft. Since these times are equal and sum to <math>9</math> hours, the trip from <math>A</math> to <math>B</math> must take half this time, or <math>4.5</math> hours. The answer is thus <math>\boxed{\textbf{D}}</math>. |
=== Solution 2 === | === Solution 2 === |
Revision as of 10:56, 16 November 2019
Problem
A power boat and a raft both left dock on a river and headed downstream. The raft drifted at the speed of the river current. The power boat maintained a constant speed with respect to the river. The power boat reached dock
downriver, then immediately turned and traveled back upriver. It eventually met the raft on the river 9 hours after leaving dock
How many hours did it take the power boat to go from
to
?
Solution
Solution 1
WLOG let the speed of the river be 0. This is allowed because the problem never states that the speed of the current has to have a magnitude greater than 0. In this case, when the powerboat travels from to
, the raft remains at
. Thus the trip from
to
takes the same time as the trip from
to the raft. Since these times are equal and sum to
hours, the trip from
to
must take half this time, or
hours. The answer is thus
.
Solution 2
What's important in this problem is to consider everything in terms of the power boat and the raft, since that is how the problem is given to us. Think of the blue arrow as the power boat and the red arrow as the raft in the following three diagrams, which represent different time intervals of the problem.
Thinking about the distance covered as their distances with respect to each other, they are distance apart in the first diagram when they haven't started to move yet, some distance
apart in the second diagram when the power boat reaches
, and again
distance apart in the third diagram when they meet. Therefore, with respect to each other, the boat and the raft cover a distance of
on the way there, and again cover a distance of
on when drawing closer. This makes sense, because from the 1st diagram to the second, the raft moves in the same direction as the boat, while from the 2nd to the 3rd, the boat and raft move in opposite directions.
Let denote the speed of the power boat (only the power boat, not factoring in current) and
denote the speed of the raft, which, as given by the problem, is also equal to the speed of the current. Thus, from
to
, the boat travels at a velocity of
, and on the way back, travels at a velocity of
, since the current aids the boat on the way there, and goes against the boat on the way back. With respect to the raft then, the boat's velocity from
to
becomes
, and on the way back it becomes
. Since the boat's velocities with respect to the raft are exact opposites,
and
, we therefore know that the boat and raft travel apart from each other at the same rate that they travel toward each other.
From this, we have that the boat travels a distance at rate
with respect to the raft both on the way to
and on the way back. Thus, using
, we have
, and to see how long it took to travel half the distance, we have
Solution 3
Let be the time it takes the power boat to go from
to
in hours,
be the speed of the river current (and thus also the raft), and
to be the speed of the power boat with respect to the river.
Using , the raft covers a distance of
, the distance from
to
is
, and the distance from
to where the raft and power boat met up is
.
Then, . Solving for
, we get
, which is
.
See also
2011 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 11 |
Followed by Problem 13 |
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 AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.