Mike J answered First, find a common denominator for 5 and 15:5, 10, 1515 Next, change 7 4/5 --> 7 and 12/15 Now, subtract:18 and 6/15 - 7 and 12/15 must be changed, borrow one from 18:17 and 21/15 - 7 and 12/15 = 10 and 9/15 --> 10 and 3/5 Your answer is 10 and 3/5