Number of hours A required do a piece of work: 12
Number of hours B required do a piece of work : 15
Work done by A in one hour: \(\frac{1}{12}\)
Work done by B in one hour: \(\frac{1}{15}\)
Work done by A and B together in one hour: \(\frac{1}{12}+\frac{1}{15}=\frac{9}{60}=\frac{3}{20}\)
∴ They can do the work together in \(\frac{20}{3}\) hours = \(6\frac{2}{3}\) hours.