Interest equation question

[Guest]

Hi, I'm trying to write an equation to solve this problem and running into difficulty:

I have 300000 to invest. A safe investment earns 3.5% per year. A risky investment yields 7% a year. How much is needed to be invested at each rate to earn $15,000 per year.

I'm certain it has something to do with the simple interest formula but I just can't make this work.

[Guest]

Suppose you invest x money in the safe fund.
So the investment in the risky one is 300000 - x

3.5% = 0.035
7% = 0.07

You need to get 15 000 per year so:
0.035x + 0.07(300000 - x) = 15 000
0.035x + 21000 - 0.07x = 15000
-0.035x = -21000 + 15000
-0.035x = -6000
0.035x = 6000
x = 171428.57
So you have to invest 171428.57 in the safe fund and the rest in the risky one.

[leesajohnson]

3.5% = 0.035
7% = 0.07

0.035x + 0.07(300000 - x) = 15 000
solving this we will get
x = 171428.57