A student driving to university must pass through seven sets of traffic lights. Suppose it is known that each of the traffic lights is red 35% of the time and that all lights function independently. What is the probability that the student will have to stop at two or more lights on her way to university?

I believe it's 5/7 x .35 giving you a 25% chance.



double check this though

This post was edited by Welterweight on Apr 11 2013 09:53pm

~76.62%

Use the binomial law n=7, p=0.35

Stop at exactly 1 red light : 7*0.35^1*0.65^6
Stop at exactly 0 red light : 0.65^7

Stop at 2 or more red lights : 1 - sum of previous probabilities

This is exactly the same problem that your motorists and seatbelts problem.

But this time it's easier to do the exact calculation.

And I see now where you got 0.8708, it's just an approximative answer given by the normal law (as Exx pointed out).