Selling price = cost+profit, that is, a=x+20%x,

a= 1.2x

x=a/ 1.2

The profit increased to 30%, that is, from 10% to10% x = 0.1* a/1.2 = a/12.

Under the condition of constant cost, increasing profit is equivalent to increasing the selling price, which is a/ 12 yuan.