Решите уравнения; b: 25/6=4/7: 20/21

B : 25/6 = 4/7 : 20/21
b * 6/25 = 4/7 * 21/20
b* 6/25 = 4*21/7*20
b * /25 = 1*3/1*5
b * 6/25 = 3/5
b = 3/5 : 6/25
b = 3/5 * 25/6
b = 3*25/5*6
b = 1*5/1*2
b = 5/2

проверка:
5/2 : 25/6 = 4/7 : 20/21
5/2 * 6/25 = 4/7 * 21/20
5*6 / 2*25 = 4*21 / 7*20
1*3 / 1*5 = 1*3 / 1*5
3/5 = 3/5