1) 21 * 1/49 - 15 * 1/7 = 21/49 - 15/7 = 21/49 - 105/49 = - 84/49 = - 12/7
2) (9/26 - 22/26) * 13/8 = - 13/26 * 13/8 = -13/2 * 1/8 = - 13/16
3) - 250 * 20 = - 5000
4) (4/10 - 7/10) : 1/4 + 20/10 = - 3/10 : 1/4 + 20/10 = - 3/10 * 4/1 + 20/10 = -12/10 + 20/10 = 8/10 = 4/5
5) 18 - 9х = 4 - 5х + 5
-9х + 5х = 4 - 5 - 18
- 4х = - 19
х = 19/4