The answer process is as follows:

Set the sales volume to increase by x compared with the original price.

( 1-20%)( 1+x)= 1

80%+80%x= 1

80%x= 1-80%

80%x=20%

x=20%÷80%

x=25%

Extended data:

Law of integer division

(1) Start with the high order of the dividend, first look at how many digits there are in the dividend, and then try to divide the dividend by the first few digits. If it is smaller than the divisor, divide it by one digit as much as possible;

(2) Write the quotient on ex-dividend;

(3) The remainder after each division operation must be less than the divisor.

Methods to solve this kind of application problems:

(1) Carefully examine the question, find out the meaning of the question, find out the unknown quantity, and set it as the unknown quantity.

(2) Find out the equivalence relation in the problem and list the equations.

(3) Solve the equation correctly.

(4) inspection.