15 3/28-(4 11/28+у) = 7 - 1 9/28
15 3/28-(4 11/28+у) = 7 - 1 9/28; 15 3/28-4 11/28-у=7-1 9/28; 10 31/28-11/28-у=7/1-37/28; 10 20/28-у=196/28-37/28; 10 20/28-у=159/28; У=300/28-159/28; У=141/28; У=5 1/28