Quantitative Aptitude

Pipes A and B can fill an industrial vat in 20 hours and 30 hours respectively, while Pipe C empties it in 40 hours. If all three pipes are opened together, the vat will be completely filled in:

Options:

• (A) $17\frac{1}{7}$ hours
• (B) $16\frac{2}{3}$ hours
• (C) 15 hours
• (D) $18\frac{2}{5}$ hours