1. 90 = 2 * 3^2 * 5
2. 1 1/4 = 5/4 = 125/100 = 1,25
3.1/5 + 3/4 = 4/20 = 15/20 = 19/20
4. 3x = 18
x = 6
5. 13 - (-7) = 13 + 7 = 20
6. 0/6 * (-0,3) = - 0,18
7. x/12 = 2/3
3x = 24
x = 8
8. -4 3/7; 0; 0,1239; 0 ,126
9. 3 7/10 - 5 5/12 = - 2 (7/10 - 5/12) = -2 (42 - 25)/60 = - 2 17/60