а) 2/7:8+5:0,7-3/4:21

а) 2/7 : 8 + 5 : 0,7 - 3/4 : 21 = 1/28 + 7 1/7 - 1/28 = (1/28 - 1/28) + 7 1/7 == 7 1/7.Пояснения:1) 2/7 : 8 = 2/7 * 1/8 = 2/56 = 1/282) 5 : 0,7 = 5 : 7/10 = 5 * 10/7 = 50/7 = 7 целых 1/73) 3/4 : 21 = 3/4 * 1/21 = (1*1)/(4*7) = 1/28