Question

a carpenter goes through 2 3/5 boxes of nails finishing 3 1/2 rooves. How much would he use finishing 8 roves?

Answer 1

Find the unit rate first, and then multiply it by 8:

2 3/5 boxes
---------------- = 13/5 divided by 7/2.
3 1/2 roofs

You can obtain the correct result by inverting the divisor, 7/2, and then multiplying:

13/5 times 2/7.   This comes out to 26/35.  That represents 26/35 box of nails per roof.

To do 8 roofs (not roves), you'd need (26/35)(8/1) boxes:  5.94 boxes.  Why not buy 6 boxes to be sure you'll have enough?