20/20 is just another way of representing 1. There are 20 twentieths in 1. 3/4+1/5 sums to 19/20, so subtracting 19 twentieths from 1 is the same as subtracting 19 twentieths from 20 twentieths. The answer is 1/20, one twentieth. 1/20 is what's left over of the ham. Since there were 20 ounces of ham originally, 1/20 of 20 ounces, that is, one ounce, is all that remains.