A man sold 150 chairs and had a gain equal to selling price of 25 chairs. Find his profit percentage.

20%

According to the question, 

Let CP = cost price of 1 chair

SP = selling price of 1 chair

Profit = 25 x SP 

Total CP = 150 x CP

Profit = 150 x SP - 150 x CP = 25 x SP

150CP = 125SP

CP : SP = 5 : 6

Profit percentage = $\frac{1}{5}$ x 100 = 20%