1/ 12x+ 1/9( 10-x)= 1

3x+4( 10-x)=36

3x+40-4x=36

40-36=4x-3x

x=4

A: A did it for four days.

Analysis because it takes 12 days for A to do a project alone, that is, it takes112 for A to do a total project every day, then the speed of A is112.

If B thinks so, B's speed is 1/9. Take the total project as the unit 1. The total project of Party A and Party B is 1.

Dear, if you don't understand, just remember (1/ A days) x+( 1/ B days) (* * * days -x)= 1.

Solution: If the price is reduced by X yuan and the original book sales volume is 1, then the book sales volume after the price reduction is 1*2=2.

So available; 0.24 * 1( 1+0.5)=(0.24-X)* 2

0.36 = 0.48-2 times

X=0.06

A: The price of each book is reduced by 0.06 yuan.

0.24* 1 is the profit before the price reduction, multiplied by 1+0.5 to get the profit after the price reduction, and 0.24-X is the profit after the price reduction of each book, multiplied by his sales volume of 2 to get the profit after the price reduction, so both sides of the equation are equal.