а)1\5 - 1\30= .б) 1\30+1\42 =. в) 0,8 \ 1 - 1\9=. г) 17\ 5*4=.

а) 1/5 - 1/30 = 6/30 - 1/30 = 5/30 = 1/6б) 1/30 + 1/42 = 7/210 + 1/210 = 8/210 = 4/105в) 0, 8 - 1/9 = 8/10 - 1/9 =  72/90 - 10/90 = 62/90 = 31/15г) 17/5 * 4/1 = 68/5 = 13.6