Money is divided in the ratio of amount of work done or in inverse ratio of number of days taken to do the work.
Let, the boy take b days to complete work.
\(\Rightarrow\;\)\(\frac{1}{7} + \frac{1}{8} + \frac{1}{b} = \frac{1}{3}\)
\(\Rightarrow\;\)\(\frac{1}{b} = \frac{11}{168}\)
\(\Rightarrow\;\)\(b = \frac{168}{11}\)
Ratio of days = \(7:8:\frac{168}{11}\)
Amount of Rs. 1400 is divided in inverse ratio:
\(\frac{1}{7}:\frac{1}{8}:\frac{11}{168} = 24:21:11\)
So the amount would be 600 , 525, 275 rupees.