#### Answer

The family earns $\$405$ in interest after 9 months.

#### Work Step by Step

We know that the Jidobu family invests $\$18000$ for 9 months at 3%. We find the interest earned using the interest formula:
$I=Prt$
$I=18000*0.03*\frac{9}{12}$
(We convert 9 months to a fraction of a year by dividing by 12.)
$I=540*\frac{9}{12}$
$I=\$405$
Thus the family earns $\$405$ in interest after 9 months.