Correct. You can use this system to solve the problem for t, time:

$\displaystyle{d}={30}{t}\usepackage{cmbright}\Huge\$

$\displaystyle{35}-{d}={40}{t}\usepackage{cmbright}\Huge\$

Solving for t by either substitution or elimination gives t = 0.5 (in hours), which is 30 minutes.