Given,
∵ Earning of 12 boys in 7 days = Rs.840
∴ Earning of 12 boys in 1 day =Rs.\(\frac{840}{7}\) =Rs.120
∴ Earning of 1 boy in 1 day \(=\frac{120}{12}\)=Rs.10
∴ Earning of 1 boy in 6 days \(={10}\times{6}=Rs.60\)
Hence, Earning of 15 boys in 6 days \(={60}\times{15}=\)Rs.900
