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.