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2007 AMC 12B Question 13

AMC12 · 2007 AMC 12B

Question 13

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?
  1. $\dfrac{1}{63}$
  2. $\dfrac{1}{21}$
  3. $\dfrac{1}{10}$
  4. $\dfrac{1}{7}$
  5. $\dfrac{1}{3}$