By selling a stool for rupees 67.50,a carpenter loses 10%.how much percent would he gain or lose by selling it for rupees 82.50.?

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

10% gain.