A certain quantity of 40% solution is replaced by a 25% solution sch that the new concentration ratio is 35% What is the ration of solution that was replaced to the ratio of the solution that was not replaced?

You need to set up an equation that represents the word problem:

"A certain quantity of 40% solution": if we let Q = the quantity of solution, then the amount of "stuff" in this solution is 0.4Q.

"is replaced by a 25% solution": hence an amount x of the solution is removed and new 25% solution replaces it, so the total amount of "stuff" in solution is now: 0.4(Q-x)+0.25x

"such that the new concentration ratio is 35%": the total amount of liquid is still Q, but at 35% concentration, so:
0.4(Q-x)+0.25x = 0.35Q

Now you can solve for x, and find the ratio of x/Q.

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