By changing the number into a decimal, and then moving the decimal twice to the right.
You know that d = 100 and w = 125, so you plug that into the equation and solve as shown:
b= 100 + d - w
= 100 + 100 - 125
= 200 - 125
= $75
You now have $75 left as balance.
It would be 1 / (x + 2) (x - 4)
perimeter = sum of all sides
27 = 9+7.8+x
x (third side) = 10.2