What percent of water must be mixed with honey so as to gain 20% by selling the mixture at the cost price of honey?

Let cost of 1 unit of honey = ₹1. To gain 20% by selling the mixture at cost price of honey, for every ₹1 of honey we want to sell mixture worth ₹1.20 at ₹1 of honey's cost. Equivalently, if we add x units of water (free) to 1 unit of honey, the mixture is (1+x) units selling at (1+x)·1 = ₹(1+x). Cost = ₹1. Profit% = x·100 = 20%, so x = 0.20, i.e. 20% water must be added.