Hint
How many large pipes are required to drain the reservoir in 24 hours?
Answer
21 hours and 36 minutes.
Reasoning
Looking at the first clue:
in 12 hours, 6 large pipes can drain 1 reservoir
in 24 hours, 6 large pipes can drain 2 reservoirs
(*) in 24 hours, 3 large pipes can drain 1 reservoir
Looking at the second clue:
in 8 hours, 3 large + 9 small pipes can drain 1 reservoir
in 24 hours, 3 large + 9 small pipes can drain 3 reservoirs
But, by (*), we know that in those 24 hours, 3 large pipes can drain 1 of those reservoirs.
Therefore, the other 2 reservoirs can be drained by the small pipes on their own:
in 24 hours, 9 small pipes can drain 2 reservoirs
in 24 hours, 1 small pipe can drain 2/9 reservoirs
multiply the hours by 9:
in 216 hours, 1 small pipe can drain 2 reservoirs
in 216 hours, 5 small pipes can drain 10 reservoirs
divide the hours by 10:
in 21.6 hours, 5 small pipes can drain 1 reservoir
21.6 hours = 21 hours and 36 minutes.
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Puzzle 18
Can you find the country hidden in the following paragraph:
At school, most of my friends learnt French. I learnt Chinese, mainly because I planned to visit the area when I grew up.
Answers
foo + tpr + int = footprint
som + ewh + ere = somewhere
pin + eap + ple = pineapple
tec + hni + que = technique
ref + ere + nce = reference
pro + fes + sor = professor
bio + gra + phy = biography
clo + ckw + ise = clockwise
bar + ome + ter = barometer
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Puzzle 20
This BrainBashers safe has a rather unusual control panel.
In order to open the safe, you have to press every single square once only, in the correct sequence, ending on the square marked E.
Each square indicates which square to press next, for example, 2U means move up 2 squares and 3R means move right 3 squares.
Can you determine which square you must start with?
6R
4R
3D
2D
2L
2L
2D
2D
1U
4R
4D
3L
2L
2L
1U
1R
3R
3L
2U
E
2D
1R
1U
3R
1D
2D
1L
3L
4U
1L
3U
2D
2U
3U
2D
1D
1U
1D
3L
2R
3L
2U
1R
1U
2U
2R
2U
1U
2L
Note: this puzzle is not interactive, and the safe buttons cannot be clicked.