Question

$12,000 is invested for 4 years at a simple interest rate of 1.5%. How much does the investment earn?

Answer 1

You didn't say if the 1.5% was per month, per 6 months, or per year. So, I'm just gonna put it all.
1.5% per month: The interest is $8640 and the amount is $20640.
1.5% per 6 months: The interest is $1440 and the amount is $13440.
1.5% per year: The interest is $720 and the amount is $12720.

Answer 2

since 12% is 2/3 as much interest, 3/2 as much should be invested there.

Or, since the interest ratio is 2:3, the investment should be 3:2

So, 40% at 18% and 60% at 12%

.12 * 7200 = .18 * 4800 = 864

or, in the usual algebraic way,

.12x = .18(12000-x)