выполнить вычитание. 1) (2/5-3/20)-(9/10-4/5) 2) (13/18-3/8)-(5/8-1/3)

(2/5 - 3/20) - (9/10 - 4/5) = 3/20
1) 2/5 - 3/20 = (2*4 - 3*1)/20 = (8 - 3)/20 = 5/20 = 1/4
2) 9/10 - 4/5 = (9*1 - 4*2)/10 = (9 - 8)/10 = 1/10
3) 1/4 - 1/10 = (1*5 - 1*2)/20 = (5 - 2)/20 = 3/20

(13/18 - 3/8) - (5/8 - 1/3) = 1/18
1) 13/18 - 3/8  = (13*4 - 3*9)/72 = (52 - 27)/72 = 25/72
2) 5/8 - 1/3 = (5*3 - 1*8)/24 = (15 - 8)/24 = 7/24
3) 25/72 - 7/24  = (25*1 - 7*3)/72 = (25 - 21)/72 = 4/72 = 1/18