5/21 + 4/7
The easiest way to do this is to find a least common denominator. We notice that 21 is a multiple of 7, so we can change 4/7 to a fraction ?/21
If we have to multiply the denominator by 3 to get there, we also have to multiply the numerator by 3 to keep the fraction the same.
5/21 + 4(3)/7(3)
5/21 + 12/21
17/21 is your answer
