book Maths Puzzle Numerical Ability

How many ml of a 40% acid solution should be added to 100 ml of a 50% acid solution to create a 42% acid solution?

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book Other Question

If the sales of a product increased by 8% in one year and by 15% in the next year, what was the overall percentage increase in sales over the two years? Sorry if the subject is wrong!

My question is:

3x-y= -3/2
4x+y=5

My work:

4x+y=5
-4x -4x

From there I get really confused! Thanks for the help!