Basically all you need to do is to build correct proportion. Let the [tex]x[/tex] represent amount of problems solved correctly. Then:
[tex]18[/tex] problems - [tex]36%[/tex]
[tex]x[/tex] problems - [tex]64%[/tex]
Now, let's find that amount:
[tex]x= \frac{18*64}{36} = \frac{612}{36}=17[/tex]
So, Michael had solved 17 problems right and 18 problems wrong. The total amount of problems on the test was 17 + 18 = 35.
