Original contrib per person = G/10

New contribution per person = G/(10+n)

Reduction in contribution per person = G/10 - G/(10+n)

Solve this to get Gn/10(10+n)

(A)

Because there are undefined variables in the answers this question can be quickly solved even if you aren't good at algebraic formulas - you can just pick numbers and work out the problem.

if we Say that the gift cost $100 so that if 10 people were contributing they would contribute $10 each. Now if we have 5 extra people contributing then the $100 gift will cost peole $6.66. Thus the price would have been reduced by $3.34.

Then you put the same numbers you used above into the answers to determine which answer will yield the same result. if you put these numbers into A you get (100)(5)/(100+10(5) which equals 500/150 which reduces to #3.34 and is thus your answer.