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Hint: The exact size of the pipes and reservoir does not matter.
Answer: 21 hours and 36 minutes.
The exact size of the pipes and reservoir does not matter. If we label the large pipes L, the small pipes S and if the reservoir has a total of T litres, then:
T ÷ 6L = 12 which means L = T ÷ 72
and
T ÷ (3L + 9S) = 8 which means that S = T ÷ 108
we want
T ÷ 5S which gives 108 ÷ 5 hours = 21.6 hours = 21 hours and 36 minutes.
Puzzle 2
Last weekend I was given my pocket money, which is meant to last me all week.
On Monday, I spent a quarter of my money on clothes.
On Tuesday, I spent one third of my remaining money on a CD.
On Wednesday I spent half of my remaining money on sweets.
Finally on Thursday I spent my last £1.25 on a comic.
Hint: Adrian had been teaching 6 years longer than Betty.
Answer:
Adrian had been teaching 21 years.
Betty had been teaching 15 years.
Charles had been teaching 7 years.
Adrian and Betty had been teaching for a total of 36 years.
Charles and Betty had been teaching for a total of 22 years.
Charles and Adrian had been teaching for a total of 28 years.
Let Adrian = A, Betty = B and Charles = C, then:
A + B = 36 [1] C + B = 22 [2] C + A = 28 [3]
If we use [3]  [2] we have:
A  B = 6 [4]
If we use [1] + [4] we have:
2A = 42 A = 21
By [1] B = 15. By [3] C = 7.
Puzzle 4
Tommy: "How old are you, Mamma?"
Mamma: "Let me think, Tommy. Well, our three ages add up to exactly seventy years."
Tommy: "That's a lot, isn't it? And how old are you, Papa?"
Papa: "Just six times as old as you, my son."
Tommy: "Shall I ever be half as old as you, Papa?"
Papa: "Yes, Tommy; and when that happens our three ages will add up to exactly twice as much as today."
Tommy: "And supposing I was born before you, Papa; and supposing Mamma had forgot all about it, and hadn't been at home when I came; and supposing..."
Mamma: "Supposing, Tommy, we talk about bed. Come along, darling. You'll have a headache."
Now, if Tommy had been some years older he might have calculated the exact ages of his parents from the information they had given him. Can you find out the exact age of Mamma?
[Ref: ZPYU] Mamma's Age. Amusements In Mathematics by Henry Ernest Dudeney (1917).
Hint: The answer involves years and months.
Answer:
The age of Mamma must have been 29 years 2 months.
That of Papa, 35 years; and that of the child, Tommy, 5 years 10 months. Added together, these make seventy years. The father is six times the age of the son, and, after 23 years 4 months have elapsed, their united ages will amount to 140 years, and Tommy will be just half the age of his father.
The answer above is taken from the original book, here is another version of the answer:
If we call Tommy T, Mamma M and Papa P we can see that:
"our three ages add up to exactly seventy years" leads to
T + M + P = 70 (1)
"Just six times as old as you" leads to
P = 6 x T (2)
In an unknown number of years (Y) "Shall I ever be half as old as you" leads to:
P + Y = 2 x (T + Y) (3)
and "our three ages will add up to exactly twice as much as today" leads to:
(T + Y) + (M + Y) + (P + Y) = 140
which can be written as
T + M + P + 3Y = 140 (4)
We can see from (4) and (1) that
3Y = 70
so
Y = 70 ÷ 3 (5)
Using (2) and (5) in (3) we have
P + Y = 2 x (T + Y)
6 x T + 70÷3 = 2 x (T + 70÷3)
4 x T = 70÷3
T = 70÷12 (6)
We can now use (6) in (2) to see that:
P = 6 x T
P = 6 x 70÷12
P = 70÷2
And using the values for T and P in (1) we have:
T + M + P = 70
70÷12 + M + 70÷2 = 70
Multiply throughout by 12 to give:
70 + 12 x M + 420 = 840
12 x M = 840  420  70
12 x M = 350
M = 350÷12
So: Tommy = 70÷12 = 5.83333 = 5 years 10 months.
Papa = 70÷2 = 35 = 35 years.
Mamma = 350÷12 = 29.1666 = 29 years 2 months.
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