Question

Michael had 18 problems wrong on his test.His score is 64% correct.How many problems were on the test

Answer 1

Basically all you need to do is to build correct proportion. Let the $x$ represent amount of problems solved correctly. Then:
$18$ problems - $36%$
$x$ problems - $64%$

Now, let's find that amount:
$x= \frac{18*64}{36} = \frac{612}{36}=17$

So, Michael had solved 17 problems right and 18 problems wrong. The total amount of problems on the test was 17 + 18 = 35.