1) 4 6/7 -1 13/14=
2) 2 2/3 * 6 /16=
3) 3/8 от 4/21=
4) 15 процентов от 20/21

20/21 * 15 ÷ 100 = 100/7 ÷ 100 = 100/7 * 1/100 = 1/72 2/3 * 6/16 = 8/3 * 3/8 = 14 6/7 - 1 13/14 = 34/7 - 27/14 =(34*2 - 27*1)/14 = (68 - 27)/14 = 41/14 = 2 13/143/8 * 4/21 = 1/14