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%?
In order to reach an above 90% on-time departure rate, the airport will need to have 10 out of 11 flights depart on time, so 7 more is the answer. If only 6 more depart on-time, it will be 90% exactly, and that's not enough.
The correct answer is B.
I am not sure why number 11 was chosen to solve the problem ? why other random numbers were no considered solve the above one? Please explain
9/10 = 90%
but we want over 90%
10/11 > 90%