Twenty workers can build a 240 m long boundary wall in 18 days. How many workers are needed to build a 400 m wall of the same kind in 15 days?

Explanation

1. 01

   Work per metre = $(20 \times 18) \div 240 = 1.5$ worker-days.

2. 02

   A 400 m wall needs $400 \times 1.5 = 600$ worker-days.

3. 03

   In 15 days: $600 \div 15 = 40$ workers.