14 - 13,2 : ( 3 11/12 - 2 4/15)

14 - 13,2 : (3 11/12 - 2 4/15) = 6

1) 3 11/12 - 2 4/15 = 3 55/60 - 2 16/60 = 1 39/60
2) 13,2 : 1 39/60 = 13,2 : 99/60 = 13,2 * 60 : 99 = 8
3) 14 - 8 = 6

Пояснения:
11/12 = (11*5)/(12*5) = 55/60 - доп.множ.5
4/15 = (4*4)/(15*4) = 16/60 - доп.множ.4
1 39/60 = (1*60+39)/60 = 99/60