Difference between revisions of "1987 AIME Problems/Problem 10"
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Al walks down to the bottom of an escalator that is moving up and he counts 150 steps. His friend, Bob, walks up to the top of the escalator and counts 75 steps. If Al's speed of walking (in steps per unit time) is three times Bob's walking speed, how many steps are visible on the escalator at a given time? (Assume that this value is constant.) | Al walks down to the bottom of an escalator that is moving up and he counts 150 steps. His friend, Bob, walks up to the top of the escalator and counts 75 steps. If Al's speed of walking (in steps per unit time) is three times Bob's walking speed, how many steps are visible on the escalator at a given time? (Assume that this value is constant.) | ||
== Solution == | == Solution == | ||
| − | {{ | + | Let the total number of steps be <math>x</math>, the speed of the escalator be <math>e</math> and the speed of Bob be <math>b</math>. |
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| + | In the time it took Bob to climb up the escalator he saw 75 steps and also climbed the entire escalator. Thus the contribution of the escalator must have been an addition <math>x - 75</math> steps. Since Bob and the escalator were both moving at a constant speed over the time it took Bob to climb, the [[ratio]] of their distances covered is the same as the ratio of their speeds, so <math>\frac{e}{b} = \frac{x - 75}{75}</math>. | ||
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| + | Similarly, in the time it took Al to walk down the escalator he saw 150 steps, so the escalator must have moved <math>150 - x</math> steps in that time. Thus <math>\frac{e}{3b} = \frac{150 - x}{150}</math> or <math>\frac{e}{b} = \frac{150 - x}{50}</math>. | ||
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| + | Equating the two values of <math>\frac{e}{b}</math> we have <math>\frac{x - 75}{75} = \frac{150 - x}{50}</math> and so <math>2x - 150 = 450 - 3x</math> and <math>5x = 600</math> and <math>x = 120</math>, the answer. | ||
== See also == | == See also == | ||
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{{AIME box|year=1987|num-b=9|num-a=11}} | {{AIME box|year=1987|num-b=9|num-a=11}} | ||
| + | [[Category:Intermediate Algebra Problems]] | ||
Revision as of 11:08, 11 February 2007
Problem
Al walks down to the bottom of an escalator that is moving up and he counts 150 steps. His friend, Bob, walks up to the top of the escalator and counts 75 steps. If Al's speed of walking (in steps per unit time) is three times Bob's walking speed, how many steps are visible on the escalator at a given time? (Assume that this value is constant.)
Solution
Let the total number of steps be 
, the speed of the escalator be 
and the speed of Bob be 
.
In the time it took Bob to climb up the escalator he saw 75 steps and also climbed the entire escalator. Thus the contribution of the escalator must have been an addition 
steps. Since Bob and the escalator were both moving at a constant speed over the time it took Bob to climb, the ratio of their distances covered is the same as the ratio of their speeds, so 
.
Similarly, in the time it took Al to walk down the escalator he saw 150 steps, so the escalator must have moved 
steps in that time. Thus 
or 
.
Equating the two values of 
we have 
and so 
and 
and 
, the answer.
See also
| 1987 AIME (Problems • Answer Key • Resources) | ||
| Preceded by Problem 9 |
Followed by Problem 11 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
| All AIME Problems and Solutions | ||
