You can model this with the equation 4x+3=15 since it is a number (x) times 4 with 3 added and equals 15.
Subtract 3 from both sides
4x=12
Divide both sides by 4
x=3
So according to the question statements
6% of the $40k and 7$ of the second $40k and 9% ammount of the $80k
the answer would be
40000 * 0.06
+ 40000 * 0.07
+ 25000 * 0.09
= 270250