Bunuel wrote:

The first flight out of Phoenix airport had a late departure. If the next three flights departed on-time, how many subsequent flights need to depart from Phoenix on-time, for the airport's on-time departure rate to be higher than 90%?

A. 6

B. 7

C. 9

D. 10

E. 11

Here's my take on this:

Current stat:

1 on time

3 late

current % of on time departure: 3/4 = 75%

We need >90%, hence let's keep adding more flights that departed on time.

(3+1)/(4+1) = 4/5 = 80% not over to 90%

(3+2)/(4+2) = 5/6 not over to 90%

(3+3)/(4+3) = 6/7 not over to 90%

(3+4)/(4+4) = 7/8 not over to 90%

(3+5)/(4+5) = 8/9 not over to 90%

(3+6)/(4+6) = 9/10 = 90% not over to 90%

(3+7)/(4+7) = 10/11 just over to 90%

Hence 7 is the answer.