Question 240584
Hello, 
Let's start by showing x for the 4th test. So now we can set up and equation. 
(x+87+81+88)/4=87  OK, now let's multiply by 4 to remove the fraction. 
x+87=81+88=348 Combine like terms
x+256=348   Subtract 256 from both sides to get:
x=92
There we go. Needs a score of 92 to get and average of 87.
Plug it in to see if it works. 
RJ Toftness
www.math-unlock.com