The R students in a class agree to contribute equally to buy their teacher a birthday present that costs y...

Birthday present costs y dollars
Contribution of each students = $$\frac{y}{R}$$
If x of the students failed to contribute their share, then contribution of the remaining students
=
$$\frac{y}{R-x}$$
The additional number of dollars that each students must contribute in order to pay for the y dollars percent
$$=\frac{y}{R-x}-\frac{y}{R}$$
$$=\frac{yR-y\left(R-x\right)}{R\left(R-x\right)}$$
$$=\frac{yR-yR+yx}{R\left(R-x\right)}$$
$$=\frac{yx}{R\left(R-x\right)}$$ $$=\frac{yx}{R\left(R-x\right)}or\ \frac{xy}{R\left(R-x\right)}$$
$$Answer\ is\ Option\ D$$