a x b = 15
c = 16
a xb xc = 15 x 16 = 240
2.
a - b = 15 , c = 4
ac - bc = c ( a - b ) = 4 x 15 = 60
3.
a + b = 90 , b + c = 100 , a + c = 100
2a + 2 b + 2 c = 290
2 ( a + b + c ) = 290
a + b + c = 290 : 2
a + b + c = 145
90 + c = 145
c = 145 - 90
c = 55
b + c = 100
b + 55 = 100
b = 100 - 55
b = 45
a + b = 90
a + 45 = 90
a = 90 - 45
a = 45
4.
a x 3 +81 = 108
a x 3 = 108 - 81
a x3 = 27
a = 27 : 3
a = 9