I have $10,000 to invest for my daughter's car. I earned $215.04 when splitting the...

Let x be the amount invested in 3% interest so that $10000-x$ is the amount invested in 1.2% interest.
The interest earned is $215.04 so we can write:
$0.03(x)+0.012(10000-x)=215.04$
Solve for x:
$0.03x+120-0.012x=215.04$
$0.018x+120=215.04$
$0.018x=95.04$
$x=5280$
So, you invested $5280 at 3% and $\$10000-\$5280=\$4720$ at 1.2%.

Answer is given below (on video)