Пошаговое объяснение:

а) 3/4 * 5/7 = (3*5)/(4*7) = 15/28

б) 3/5 * 5/12 = 1/1 * 1/4 = (1*1)/(1*4) = 1/4

в) 5/12 * 3/20 = 1/4 * 1/4 = (1*1)/(4*4) = 1/16

г) 3/7 * 5/9 = 1/7 * 5/3 = (1*5)/(7*3) = 5/21