This is a classic average problem where we don't know all of the individual elements, but can infer what we need from the information given.

If the new average is $75 and it is 50% more than before, it must have been $50 for the 5 people contributing before John. If the average for the 5 was 50 before John, we know they contributed 250 total irrespective of the distribution.

We'll make John's contribution x so it would be the total contributions (250+x) divided by number of contributors (6) equal to the new average (75).

(250+x)/6 = 75

x = 200, John's contribution.

I go with 200..

here it is

total contribution after john = 75*6 = 450

contribution before john = 50 *5 = 250

( the average contribution size will increase by 50%, reaching $75 per person implies 75 dollars is 1.5 X. there fore X = 50)

hence john contribution = 200.

hope it helps