let 'x' represent the amount of 50% mixture used, then '20-x' represents the amount of 10% mixture used
0.25(20) = 0.50x + 0.10(20-x)
solving for 'x' we have x=7.5
20-7.5 = 12.5
7.5 pounds of 50% mixture should be combined with 12.5 pounds of 10% mixture