10/21-3/145/12-3/8, 11/12-2/9, 9/20-13/30, 5/18-7/30, 7/36-1/24, 10/27-5/18, 7/12-2/15 подводим к общему знаменателю.
10/21-3/14= 20/42-9/42=11/42;
5/12-3/8=10/24-9/24=1/24;
11/12-2/9=33/36-8/36=25/36;
9/20-13/30=27/60-26/60=1/60;
5/18-7/30=25/90-21/90=4/90;
7/36-1/24=14/72-3/72=11/72;
10/27-5/18=20/54-15/54=5/54;
7/12-2/15=35/60-8/60=27/60.