Correct. Setting up an equation that relates Mel's rate of work (1 room in 8 hours), Sid's rate of work (1 room in x hours), and the joint rate of work (3.5 rooms in 12 hours), you find $\displaystyle\frac{{1}}{{8}}+\frac{{1}}{{x}}=\frac{{3.5}}{{12}}\usepackage{cmbright}\Huge\$ . Solving for x yields x=6, so Sid can paint 1 room in 6 hours.