Question 128549
overall percent gain = (sum of separate gains) / (total amount invested)
{{{(.07*20000 + .04*50000  + .06*x) / (20000 + 50000 + x) = .05}}}
{{{(1400 + 2000 + .06*x) / (70000 + x) = .05}}}
multiply both sides by {{{70000 + x}}}
{{{3400 + .06x = .05(70000 + x)}}}
{{{3400 + .06x = 3500 + .05x}}}
{{{.01x = 100}}}
{{{x = 100/.01}}}
{{{x = 10000}}}
He must invest $10,000 at 6% to have an overall return of 5%