|Jill had the best average.
She hit 75 out of 100 pitches, for an average
of 75%. Jack hit 75 out of 100 pitches in his
first round. Then he hit 35 out of 50 pitches
in his second round. Add his hits in both rounds
(75 + 35) for a total of 110 hits. Then add the
total number of pitches Jill threw to Jack (100
in round one + 50 in round 2) for a total of 150
pitches. Finally, divide Jack's total number of
hits (110) by the total number of pitches (150)
to get his average for both rounds.
The result is 73.3%, which is lower than Jill's
average of 75%.
Source: Sterling Pocket Puzzlers,Brain Teasers, p.14