Пусть a, b, c -- искомые числа. Тогда:
(a + b + c)/3 = 20
a = 2b
c = a - 7
a + b + c = 60
b = a/2
c = a - 7
a + a/2 + a - 7 = 60
b = a/2
c = a - 7
2a + a + 2a - 14 = 120
b = a/2
c = a - 7
5a = 134
b = a/2
c = a - 7
a = 134/5 = 26 4/5
b = 134/5 : 2 = 67/5 = 13 2/5
c = 134/5 - 7 = 134/5 - 35/5 = 99/5 = 19 4/5
Ответ: 26 4/5; 13 2/5; 19 4/5.