Hi naaga, this is an algebra question. You've missed out the answers, but I'll show you how this is done.

Hopefully you can see that if fewer people pay, each has to pay more than if everyone paid? Well, the additional amount to be paid is simply:

Amount with few people paying - amount when everyone pays.

The amount when everyone pays is y/R

The amount when x fewer people pay is y/(R-x)

So the difference is:

(y/(R-x)) - (y/R)

Without seeing the answers, I can't say which one is correct, but in a similar question I have seen before, the expression needed to be changed to have a common denominator:

(y/(R-x)) - (y/R) = (yR - y(R-x)) / (R(R-x))

= y(R-R+x) / R(R-x)

= yx / R(R-x)

Of course, there may other equivalent expressions.

Whenever I see variables, I try to pick numbers and solve, so you can get a target answer to look for in the answer choices.

So I picked: R= 10, y=$20 and x=2

So originally each student contributed $ 2. If 2 students fail to contribute, the new contribution per student would be $ 2.5 (20/8), so the difference will be $ .5 USD. So the answer to look for when you substitute the values you picked is $ .5.

Hope it helps.