Question

a+b=20,b+c=30,3a+7b+4c=?

Answer 1

a + b = 20   si    b + c = 30

3 a + 7 b + 4 c = 180

= 3 a + ( 3 b + 4 b ) + 4 c =

= ( 3 a + 3 b ) + ( 4 b + 4 c ) =

= 3 × ( a + b ) + 4 × ( b + c ) =

= 3 × 20 + 4 × 30 =

= 60 + 120 =

= 180

Answer 2

a+b=20
a=10
b=10
b+c=30
10+c=30
c=20
3a+7b+4c
3×10+7×10+4×20=30+70+80=180