Correct. Setting up this problem as an equation, you find $\displaystyle\frac{{50}}{{3}}+\frac{{50}}{{h}}+\frac{{25}}{{1}}=\frac{{50}}{{1}}\usepackage{cmbright}\Huge\$ (since you add the rates of Machine A, B, and C and set that equal to the rate of all machines working together). Solving for h, you find that h=6.