1.
1 3/8 + 2 5/6 = (1 + 2) + (3/8 + 5/6) = 3 + (9/24 + 20/24) = 3 + 29/24 = 3 + 1 5/24 = 4 5/24,
2.
1/3 - 0,12 = 1/3 - 12/100 = 100/300 - 36/300 = 64/300 =16/75,
3, 4.
1,4 * 2 6/7 - 2 1/7 : 1,25 = 14/10 * 20/7 - 15/7 : 125/100 = 4 - 15/7 * 100/125 = 4 - 12/7 = 4 - 1 5/7 = 2 2/7,
5.
1/6х+х = 3,
1 1/6х = 3,
7/6х = 3,
х = 3 : 7/6,
х = 3 * 6/7,
х = 18/7,
х = 2 4/7,
6.
0,2(4х-2/3х) - 1/6х = 0,2(3 1/3х) - 1/6х = 2/10*10/3х - 1/6х = 2/3х - 1/6х = 4/6х - 1/6х = 3/6х = 1/2х,
7.
4:(20-х)=0,3,
20-х = 4 : 0,3,
20-х = 13 1/3,
х = 20 - 13 1/3,
х = 6 2/3