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Rule of Three
If it takes four men one day to build a wall, how long does it take 60,000 men to build a similar wall?
Crossing the Atlantic
Every day at noon a boat leaves Le Havre for New York while another boat leaves New York for Le Havre. The ocean crossing takes seven days. How many boats will each boat cross during their journey?
On Time
On a clock the three hands - the second, minute and hour hands - are on the same axis. How often in a 24 hour day will the second hand be parallel to either of the two other hands?
Stage: 3 Challenge Level: 
A bus route has a total duration of $40$
minutes.
Every $10$ minutes, two buses set out, one from each end.
How many buses will one bus meet on its way from one end to the other end?
See Crossing the Atlantic for a similar problem.
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NRICH is part of the family of activities in the Millennium Mathematics Project.
NRICH is part of the family of activities in the Millennium Mathematics Project.