Answer
The yearly interest combined for x dollars invested at 12% and $30,000-x$ dollars invested at 9% is $0.12x+0.09\left( 30,000-x \right)$.
Work Step by Step
According to the given conditions, we have:
The interest for x dollars invested at 12% is calculated as follows:
$x\times \frac{12}{100}=0.12x$
The interest for $30,000-x$ dollars invested at 9% is calculated as follows:
$\left( 30,000-x \right)\times \left( \frac{9}{100} \right)=0.09\left( 30,000-x \right)$
Therefore, the combined yearly interest will be $0.12x+0.09\left( 30,000-x \right)$.
