Let's first break it down to the time 8 people need for one wall. That would be 10 minutes, right? If one person would have to do it in stead of 8, you'd expect him to take 80 minutes.
To paint 7 walls, he would nee 7x80 = 560 minutes. 10 people would do that 10 times faster, ie., 56 minutes.
I know puzzles like these always have a mindtrick. Did I miss anything?? ;-)
Answer: 
<u>Move all terms to the left</u>
<u></u>
<u></u>
<u></u>
<u>Add all the numbers together, and all the variables</u>
<u></u>
<u></u>
<u></u>
<u>Multiply elements</u>
<u></u>
<u></u>
<u></u>
<u>Move all terms containing d to the left, all other terms to the right</u>
<u></u>
<u></u>
OK...... your answer would be
- 10y^2 - 26y + 1/y
H0P3 It H3LPS :)