Write each pair of fractions with a common denominator 2/4and 7/8

4/8 and 7/8 you can add them to make 11/8 or 1 3/8 but that wasn't really asked

2/4= 4/8
multiply the 4 by 2 to get 8. What you do to the bottom you also have to do to the top, so 2*2=4.

4/8 and 7/8 are common denominators. If you needed a separate common denominator for 7/8 it is below. 

7/8=14/16
Same method as above. The easiest way is to multiply by 2.