D = B x .80

A = 120 + B + (D x 2) + ((D x.80) x 2)

solving for B...

1st eliminate D

A = 120 + B + ((B x .80) x 2) + ((B x .80) x 2)

2nd eliminate inner parenthesis

A = 120 + B + (B x 1.6) + (B x 1.6)

3rd combine

A = 120 + B + 2(B x 1.6)

4th expand

A = 120 + B + 2B x 3.2

5th put together like terms

A = 3B + 123.2

and then basic algebra

3B = A - 123.2

B = (A - 123.2) / 3

and thats it!