Question 552313
Let's try this questions with some numbers in there first. Bare with me, it'll be much easier to do with the variables after we try this.

There were 100 pupils in the junior class and they decided to spend 10 dollars on a school project to which all would contribute equally. Then 20 students refused to donate. How much more did each of the rest of the students have to donate?

So the students are contributing a total of $1000 (100*10) and now just 80 are contributing. So that's $12.50 ($1000/80) for each student. So they each have to pay $2.50

So what were the calculations we did? We multiplied the number of students by their contribution, divided by the number of students minus 20 and then subtracted the original contribution 

(100*10)/80-10

In place of the numbers, let's put the variables. But how did we get 80? We did 100-20

{{{(p*d)/(p-20)-d}}}