Correct. Machine A’s rate of work is $\displaystyle\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ (1 box in 4 minutes), and Machine B’s rate is $\displaystyle\frac{{0.5}}{{3}}\usepackage{cmbright}\Huge\$ (half a box in 3 minutes). Setting up an equation, you find $\displaystyle\frac{{1}}{{4}}+\frac{{0.5}}{{3}}=\frac{{1}}{{x}}\usepackage{cmbright}\Huge\$ . Solving for x yields x=2.4, which is 2 minutes 24 seconds.