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%?
I don't understand how many total flights we have ?
we're being asked about a percent, so let's jot down the percent formula:
% = (part/whole) * 100%
In this question, the whole is the total number of flights and the part is the number that depart on time.
Of the first 4 flights, we know 1 is late and 3 are on time. So, the % on time out of the first 4 is 3/4 = 75%.
To satisfy the question's criterion, we need that on time rate to be greater than 90%. In other words:
(# on time)/(total # of flights) > 9/10
Since we only have 1 flight that's late, the quickest way to get more than 9/10 on time is to have every flight on time from here on out. Since 9/10 is too small, we need 10/11 to get greater than a 90% rate. We've had 4 flight so far, so we need 7 more flight to get to 10/11 - choose (B).
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