A certain traffic light is red for 20 seconds, yellow for 4 seconds, and green for 36 seconds. Find that probability that of the next 12 cars that randomly arrive at the light, exactly 4 will be stopped by a red light.

In statistics, this is a binomial. This is how you check.
B - binary?
I - independent?
N - trials?
S - probability success?

B - yes, stopped at red light or not
I - previous cars do not affect future cars
N - 12 trials
S - prob success is 20/60

On your calculator use the binompdf function. Only use binomcdf if you want to find the probability that four AND fewer will be stopped To find, 2nd —> vars —> binompdf

Binompdf(12 (trials), 20/60 (p), 4 (n)) = .2384

The probability that exactly 4 out of the next 12 cars will be stopped at the red light is .2384