А)1/4+1/7+9/36 Б)1/5+3/10+7/20 В)3/20+7/30+2/40

Приведем значения к общему знаменателю и вычислим:

1. 1/4 + 1/7 + 9/36 = 1/4 + 1/7 + 9/(9 * 4) = 1/4 + 1/7 + 1/4 = (1 + 1)/4 + 1/7 = 2/4 + 1/7 = 1/2 + 1/7 = (1 * 7)/14 + (1 * 2)/14 = 7/14 + 2/14 = (7 + 2)/14 = 9/14.

2. 1/5 + 3/10 + 7/20 = (1 * 4)/20 + (3 * 2)/20 + 7/20 = 4/20 + 6/20 + 7/20 = (4 + 6 + 7)/20 = (10 + 7)/20 = 17/20.

3. 3/20 + 7/30 + 2/40 = 3/20 + 7/30 + 2/(2 * 20) = 3/20 + 7/30 + 1/20 = (3 + 1)/20 + 7/30 = 4/20 + 7/30 = 4/(4 * 5) + 7/30 = 1/5 + 7/30 = (1 * 6)/30 + 7/30 = 6/30 + 7/30 = (6 + 7)/30 = 13/30.