After turning off C, rate of A + B = ( \frac{1}{6} + \frac{1}{4} = \frac{4}{24} + \frac{6}{24} = \frac{10}{24} = \frac{5}{12} ) tank/hour.

Time needed = ( \frac{\text{remaining}}{\text{rate}} = \frac{\frac{5}{12}}{\frac{5}{12}} = 1 ) hour.

In 2 hours: ( 2 \times \frac{7}{24} = \frac{14}{24} = \frac{7}{12} ) tank filled.

Remaining empty: ( 1 - \frac{7}{12} = \frac{5}{12} ) tank.

Ribbon used for small boxes: ( 0.8 \times S ) metres. Ribbon used for large boxes: ( 1.2 \times (S + 3) ) metres.