Question 1082887
<pre>
Let the fourth exam have score x.

To average numbers we add them and divide by the number of
numbers.

So we add all four together and divide by 4.  Then we set that 
equal to 77 

{{{(85+90+61+x)/4}}}{{{""=""}}}{{{77}}}

Now solve for x.

Begin by multiplying both sides by 4 to get rid of the 
denominator of 4.

You finish.

Edwin</pre>