Puzzle 5
Within the BrainBashers school, the science department has three disciplines.
In total, 280 students study chemistry, 254 students study physics, and 280 students study biology.
In total, 97 students study chemistry and physics, 138 students study physics and biology, and 152 students study chemistry and biology.
73 students study all three disciplines.
Can you determine how many students there are in the science department? The answer is not 814.
Puzzle Copyright © Kevin Stone
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Hint
How many students study chemistry and physics, but not biology?
Answer
500 students.
Reasoning
The answer is more easily seen if three intersecting circles are drawn, and the numbers inside each section are worked out.
We know that 73 students study all three disciplines, which allows us to work out the numbers.
97 students study chemistry and physics, so 97 – 73 = 24 study just chemistry and physics.
138 students study physics and biology, so 138 – 73 = 65 study just physics and biology.
152 students study chemistry and biology, so 152 – 73 = 79 study just chemistry and biology.
280 students study chemistry, so 280 – 73 – 24 – 79 = 104 study just chemistry.
254 students study physics, so 254 – 73 – 24 – 65 = 92 study just physics.
280 students study biology, 280 – 73 – 65 – 79 = 63 study just biology.
For a total of: 104 + 92 + 63 + 24 + 65 + 79 + 73 = 500 students.
Puzzle 6
A large freshwater reservoir has two types of drainage system. Small pipes and large pipes.
6 large pipes, on their own, can drain the reservoir in 12 hours.
6 small pipes, on their own, can drain the reservoir in 18 hours.
How long will 6 large pipes and 6 small pipes take to drain the reservoir?
Puzzle Copyright © Kevin Stone
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Hint
How many reservoirs can each drain in 36 hours?
Answer
7 hours and 12 minutes.
Reasoning
We can use a time that 12 hours and 18 hours both fit in to, in 36 hours:
6 large pipes can drain 3 reservoirs.
6 small pipes can drain 2 reservoirs.
Therefore, together, 6 large and 6 small pipes can drain 5 reservoirs.
36 hours ÷ 5 = 7.2 hours for one reservoir.
7.2 hours = 7 hours and 12 minutes for one reservoir.
Puzzle 7
In the following sum, the digits 0 to 9 have all been used, and the top row's digits add to 9.
Knowing that O = Odd, and E = Even (zero is even), can you determine each digit?
Puzzle Copyright © Kevin Stone
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Hint
The answer is 4 digits long, so what must G equal?
Answer
423 + 675 = 1098.
Reasoning
Remembering that:
even + even = even
odd + odd = even
even + odd = odd
To discuss individual letters, it's easiest to represent the sum as:
A B C
D E F +
————————
G H I J
A + D has to be over 9, which means that G = 1.
B + E = I, is even + odd = odd, which means that we can't have a carry from C + F (otherwise it would have been even + odd + 1, which is even).
The 1 has already gone, so the smallest possible value for either C or F is 3, which means that the other can't be 7 or 9 (otherwise we'd have a carry).
Therefore, C and F are 3 and 5, but we don't know which is which. But we do now know that J = 8.
A + D = H, is even + even = even, which means that we can't have a carry from B + E. Therefore, E can't be 9, as this would force a carry. So E = 7.
I is the only remaining odd number, so I = 9.
Which means that B = 2.
Neither A nor D can be 0 (otherwise we would have two of the same digit). So, H = 0.
Therefore, A and D are 4 and 6 (but we don't yet know which is which).
Since the top row's digits have to add to 9, A can't be 6, so A = 4, making C = 3.
This makes the sum 423 + 675 = 1098.
Puzzle 8
My current age is the age of my favourite book (which is 14 years old) plus one third of my age.
How old will I be when my favourite book is twice its current age?
Puzzle Copyright © Kevin Stone
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Hint
A little algebra might help.
Answer
I will be 35 in 14 years.
Reasoning
I am currently 21.
If X is my current age, then:
X = 14 + X
———
3
so
2X = 14
———
3
and
2X = 42
So
X = 21
I am currently 21 and my favourite book is currently 14 years old, when it is 28 years old (double its age) I will be 35.
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