Hello,

I'll use decimal numbers instead of percents:

25% = 0.25

75% = 0.75

40% = 0.4

In the original bag are 20 * 0.25 = 5 pounds of pure cement.

Let x be the amount of pure cement which has to be added. Then you get the proportion:

Add 5 pounds of pure cement to this mixture and then it contains 40% of cement.

EB