Пошаговое объяснение:

а)

3/20 + 7/25 = (3*5)/(20*5) + (7*4)/(25*4) =

15/100 + 28/100 = (15+28)/100 = 43/100

б)

7/9 - 1/4 = (7*4)/(9*4) - (1*9)/(4*9) =

28/36 - 9/36 = (28-9)/36 = 19/36

в)

1/3 + 1/6 + 1/18 = (1*6)/(3*6) + (1*3)/(6*3) + 1/18 =

6/18 + 3/18 + 1/18 = (6+3+1)/18 = 10/18 =

(10:2)/(18:2) = 5/9

г)

1/3 - 2/9 + 5/6 = (1*6)/(3*6) - (2*2)/(9*2) + (5*3)/(6*3) = 6/18 - 4/18 + 15/18 = (6-4+15)/18 = 17/18