mixtures - very tough

just looking at the numbers alone, i'd say the answer is 1/2(="c") b/c only when 1/2 of 50% solution is mixed with 1/2 of 30% solution, one will get a whole of 40% solution.

vkb16-

I hope this explanation helps. Let's suppose our 50% acid solution is A and our 30% solution is B.

"Some part of a 50% solution of acid was replaced with an equal amount of 30% solution of acid..." should translate to...
A - xA + xB (where x is the amount replaced)
Basically, we're replacing x amount of solution A with x amount of solution B (as the question states "equal amount").

We know that the new solution is 40%, so:
A - xA + xB = .4
Substitute back in our original values
.5 -.5x +.3x = .4
-.5x +.3x = -.1
-.2x = -.1
x = 1/2

C.

is there a way of plugging in numbers and solving it. i tried, but couldn't get it. could someone please let me know?

another method to solve such q is attached