Work and wages problem

Efficiencies of a man, woman, and child are $3x, 2x, x$.

Daily wage of 20 men, 30 women, and 36 children = $20(3x) + 30(2x) + 36(x) = 156x = 78 \Rightarrow x = 0.5$.

Daily wage of 15 men, 21 women, and 30 children = $15(3x) + 21(2x) + 30(x) = 117x = 117 \times 0.5 = 58.5$.

For 18 weeks ($18 \times 7 = 126$ days): Wages = $58.5 \times 126 = 7371/-$.