I'm pretty sure $39.31 was the original sales cost because if you use proportions to find out what is 7.2% of 42.35 ( what you get you you subtract 45 by 2.65). Your equation would be x/42.35 = 7.2/100. 42.35 is the whole because tax is adding money onto your original cost, making that the part. When you cross multiply you get 100x = 304.92. Then you divide by 100 on both sides to isolate the variable, to get x = 3.04. But you don't stop there. Since 3.04 is only the 7.2%, you have to subtract that by 42. 35 to get a sum of 39. 31 as your total.
The Numerator is 1, which is the top. The Denominator is the 8. I like to remember (D)own is the (D)enominator.
You multiply 86 by 42.5 because each board is 42.5 and there are 86 boards.
