Algebra solution concentration

Question

a chemist has 2 large containers of sulfuric acid solution with concentrations of acid 15% and 30%. how many ml of each solution should he mix to prepare 1250 ml of solution with 25% acid concentration? 

Answer 1

(x ml)(15%) + (1250 ml - xml)(30%) = (1250 ml)(25%)

15x + 37,500 - 30x = 31,250

-15x = -6250

x = 416.7 ml of 15%

1250 ml - 416.7 ml = 833.3 ml of 30%