3/5а + 2/3а = 3 целых 4/5
9/15а + 10/15а = 19/5
19/15а = 19/5
а = 19/5 : 19/15 = 19/5 * 15/19 = 15/5
а = 3
Общий знаменатель 15
15 : 5 = 3 - доп.множ. к 3/5 = (3*3)/(5*3) = 9/15
15 : 3 = 5 - доп.множ. к 2/3 = (2*5)/(3*5) = 10/15
3 целых 4/5 = (3*5+4)/5 = 19/5