# Conditional Probability Traffic light question

1. Oct 13, 2005

### Pure Canuck

Hey guys Me and my friend just got this question and it seems easy but i just want to make sure we are right anyway here it is:

A road has two stoplights at consecutive intersections. The prob. of a red at the first is 0.55 and the probability of a green at the second, give a green at light oine is .75

Find the prob of a green at both intersections:
Givens = (r1 = .55) (g2|g1=.75)
now i believe its simply as just r1*.75.

which is like .33

Anybody know if i did this right?

Thanks

2. Oct 13, 2005

### *melinda*

$P(r_1 )=0.55$ and $P(g_2 |g_1 )=0.75$

but I don't know how you got your answer because .55*.75 = .4125, though I do agree that .33 is very close to the answer, depending on how you round the decimal.

3. Oct 14, 2005

### iNCREDiBLE

Hint: http://mathworld.wolfram.com/ConditionalProbability.html" [Broken].
Just use the formula.

Last edited by a moderator: May 2, 2017