If 72 boys can build a wall of 280 m length in 21 days, how many boys would take 18 days to build a similar type of wall of length 200 m?

Given:

72 boys can build a wall of 280 m length in 21 days.

Formula used:

Work = Number of Boys × Number of Days

Calculation:

Work done by 72 boys in 21 days = 280 m

⇒ Work done by 1 boy in 1 day = \(\frac{280}{72 \times 21}\)

⇒ Work done by 1 boy in 1 day = \(\frac{280}{1512}\)

⇒ Work done by 1 boy in 1 day = \(\frac{5}{27} \) m

For 200 m wall in 18 days:

⇒ Required Work = 200 m

⇒ Number of Boys × 18 days = 200 m

⇒ Number of Boys = \(\frac{200}{18 \times \frac{5}{27}}\)

⇒ Number of Boys = \(\frac{200 \times 27}{18 \times 5}\)

⇒ Number of Boys = \(\frac{5400}{90}\)

⇒ Number of Boys = 60

∴ The correct answer is option (4).