Correct. Setting up the following system:

$\displaystyle{528}-{e}={12}\cdot{t}\usepackage{cmbright}\Huge\$

$\displaystyle{e}={9.75}\cdot{\left({50}-{t}\right)}\usepackage{cmbright}\Huge\$

you find that Asa worked for 18 hours, and Bella worked for 32 hours.