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5 skilled workers can build a wall in 20 days; 8 semi-skilled workers can build a wall in 25 days; 10 unskilled workers can build a wall in 30 days. If a team has 2 skilled, 6 semi-skilled and 5 unskilled workers, how long will it take to build the wall?

1. 20 days
2. 18 days
3. 16 days
4. 15 days

Correct answer: 15 days

Solution

Per-worker rates: skilled 1/(5*20)=1/100, semi 1/(8*25)=1/200, unskilled 1/(10*30)=1/300 walls/day. Team rate = 2/100 + 6/200 + 5/300 = 6/300+9/300+5/300 = 20/300 = 1/15. So the wall takes 15 days, not 18. Correct option: '15 days'.

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