Question

The route used by a certain motorist in commuting to work contains two intersections with traffic signals. The probability that he must stop at the first signal is 0.45, the analogous probability for the second signal is 0.5, and the probability that he must stop at at least one of the two signals is 0.9.
Required:
a. What is the probability that he must stop at both signals?
b. What is the probability that he must stop at the first signal but not at the second one?
c. What is the probability that he must stop at exactly one signal?

Answer 1

Answer: a. 0.05

b. 0.40

c. 0.85

Step-by-step explanation:

Let F= Event that a certain motorist must stop at the first signal.

S =  Event that a certain motorist must stop at the second signal.

As per given,

P(F) = 0.45 , P(S) = 0.5 and P(F or S) = 0.9

a. Using general probability formula:

P(F and S) =P(F) + P(S)-  P(F or S)

= 0.45+0.5-0.9

= 0.05

∴ the probability that he must stop at both signals = 0.05

b. Required probability = P(F but (not s)) = P(F) - P(F and S)

= 0.45-0.05= 0.40

∴ the probability that he must stop at the first signal but not at the second one =0.40

c. Required probability = P(exactly one)= P(F or S) - P(F and S)

= 0.9-0.05

= 0.85

∴   the probability that he must stop at exactly one signal = 0.85