Question 1203605
<br>
First a quick and easy informal solution method....<br>
You are mixing 3.5% interest investments with 5% interest investments to get an overall 4% interest.<br>
4% is twice as close to 3.5% as it is to 5%, so the amount invested at 3.5% must be twice the amount invested at 5%.<br>
Since $9000 was invested at 3.5%, the amount invested at 5% has to be half of that, which is $4500.<br>
ANSWER: $4500<br>
And a standard formal algebraic solution....<br>
3.5% of $9000, plus 5% of x, equals 4% of ($9000+x):<br>
{{{.035(9000)+.05x=.04(9000+x)}}}
{{{315+.05x=360+.04x}}}
{{{.01x=45}}}
{{{x=4500}}}<br>