exam Maths Puzzle Numerical Ability

This is im my 8th grade homework and i need help figuring it out how to do the problem...

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exam Other Question

What quantity x of a 75% acid solution must be mixed with a 25% solution to produce 500 ml of a 55% solution?

I have tried these equations
.75x+.25(.55*500)=500(.55) if we find medians of the triangle why we take 2:1