Okay, so you have two cakes cut into 21 pieces, correct? Now you choose 4 pieces from one cake, and 5 from the other. You put these pieces onto a plate. You now have 9 of 21 pieces of cake (9/21).
This can also be simplified to 3/7
Basically, you just add the numerators together when you are adding fractions and the denominator is the same. (eg 3/9 + 5/9 = 8/9)
Hope this helps!
- Z
