Lisa spends 1/5 of an hour doing her math homework and 1/3 of an hour doing her social studies homework. what fraction of an hour does she spend doing her math and social studies homework?

To solve this, all you have to do is add 1/5 and 1/3.
Well, you might be thinking, "But the denominators are not the same! Fractions have to have like denominators in order to be added together!" And you are right. So, all we have to do, is make them both have the same denominator. To do this, we have to multiply both the numerator and denominator by the same number to find an equivalent fraction.
For 1/5, we can multiply both the numerator and denominator by 3, to get 3/15. 
Likewise, we can multiply both the numerator and denominator of 1/3 by 5, to get 5/15.
Now, you can easily add 3/15 and 5/15 because all you have to do is add the numerators, because the denominators are the same!
3/15+5/15=8/15, so she spends 8/15 hour on her math and social studies homework.
Hope I helped!