1) x+15 = 55
x = 55 - 15
x = 40 grade
2) x - 10 = 91
x = 91 + 10
x = 101 grade
3) x + 36 + x = 180
2x + 36 = 180
2x = 180 - 36 = 144
x = 144 :2
x = 72 grade
4) x + 18 + x - 12 = 180
2x + 6 = 180
2x = 180 - 6 = 174
x = 174 : 2
x = 87 grade
5) 3x + 40 = x + 50
3x - x = 50 - 40
2x = 10
x = 5 grade