What quantity of 70 per cent acid solution must be mixed with a 25 per cent solution to produce 432 mL of a 50 per cent solution?

We know that x ml of 70% solution + (432-x) ml of 25% solution yields 432 ml of 50% solution. So...

0.70 (x) + 0.25 (432-x) = 0.50 (432)

0.70 x + (108 - 0.25 x) = 216

0.70 x + 108 - 0.25 x = 216

0.45 x = 108

x = 240

So 240 ml of 70% solution + 192 ml (432-240) of 25% solution yields 432 ml of 50% solution.

Check: (240 * 0.70) + (192 * 0.25) = (432 * 0.50)