1.5x+15=75
5x=75-15
x=60÷5
x=12
2.3/5x-1/4x=7/10
(3/5-1/4)x=7/10
7/20x=7/10
x=7/10÷7/20
x=2
3.120÷(1/5-1/8)=120÷3/40=1600