Question 994963
let x be the number of pounds of 4%. That means there are 500-x pounds of 20%

{{{0.14 * 500 = 0.04*x + 0.20*(500-x)}}}
{{{70 = 0.04x + 100 - 0.2x}}}
{{{-30 = -0.16x}}}
{{{187.5 = x}}}
So there are 187.5 pounds of 4%
Which leaves 312.5 pounds of 20%

As a quick check, realize that you added more 20% than you did 4%. Since 14% is closer to 20% than it is to 4%, adding more of the 20% makes sense right!