2007 AMC 12B Problems/Problem 13

Problem

A traffic light runs repeatedly through the following cycle: green for $30$ seconds, then yellow for $3$ seconds, and then red for $30$ seconds. Leah picks a random three-second time interval to watch the light. What is the probability that the color changes while she is watching?

$\mathrm{(A)}\ \frac{1}{63} \qquad \mathrm{(B)}\ \frac{1}{21} \qquad \mathrm{(C)}\ \frac{1}{10} \qquad \mathrm{(D)}\ \frac{1}{7} \qquad \mathrm{(E)}\ \frac{1}{3}$

Solution

The traffic light runs through a $63$ second cycle.

Letting $t=0$ reference the moment it turns green, the light changes at three different times: $t=30$ , $t=33$ , and $t=63$

This means that the light will change if the beginning of Leah's interval lies in $[27,30]$ , $[30,33]$ or $[60,63]$

This gives a total of $9$ seconds out of $63$

$\frac{9}{63} = \frac{1}{7} \Rightarrow \mathrm{(D)}$

2007 AMC 12B (Problems • Answer Key • Resources)
Preceded by Problem 12	Followed by Problem 14
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.

Page

Toolbox

Search

2007 AMC 12B Problems/Problem 13

Problem

Solution

See Also