a) ab + ac = 520
a x ( b + c ) = 520
a x 52 = 520
a = 10
b) ab + ac = 45
a x ( b + c ) = 45
a x 9 = 45
a = 5
c) ab + ac + 70 = 170
a x ( b + c ) = 170 - 70
25 x ( b + c ) = 100
b + c = 4
d) ab + ac - 48 = 2
a x ( b + c ) = 48 + 2
10 x ( b + c ) = 50
b - c = 5
Succes !!!