Linear system word problem

Question

A chemical storeroom has a 90% acid solution and a 40% acid solution. How many centiliters of the 90% solution should be added to the 50 centiliters of the 40% solution to yield a 50% solution?

Answer 1

Let x centiliters fo the 90% solution should be added.

(0.9x+50*0.4)/ (x+50) = 0.5

0.9x+20=0.5x+25

0.4x=5

x=12.5