Let the actual value of the stool be x.
So if the carpenter sells the stool x rupees he neither loses, now wins anything.
67.50 is the value x - 10% of x, that is:
67.50=x-0.1x
67.50=0.9x
x=67.50/0.9=75 (rupees) is the actual value of the stool.
Selling the stool for 82.50 rupees, the carpenter clearly would make a profit, say of y% of that actual value, that is y%*75.
So using the same method:
82.50=75+y%*75
y%*75=82.50-75=7.5
we can make the calculations, but clearly 7.5 is 10% of 75
Answer: 10% gain.
