А) 3/4 - (7/8 - a) = 1/8
6/8 - 1/8 = 7/8 - a
5/8 = 7/8 - a
a = 7/8 - 5/8 = 2/8 = 1/4
б) (5/6 - n) - 1/3 = 5/12
10/12 - n = 4/12 + 5/12
10/12 - n = 9/12
10/12 - 9/12 = n = 1/12
в) 3/4 - (x + 1/12) = 1/5
45/60 - 12/60 = x + 5/60
x + 5/60 = 33/60
x = 33/60 - 5/60 = 28/60 = 7/15
г) 2/3 - (x - 1/21) = 2/7
14/21 - 6/21 = x - 1/21
x - 1/21 = 8/21
x = 9/21 = 3/7