Mathematical Puzzles 

Workings

Hint
It might be easier to work backwards.

Answer
£5.00.

Reasoning
Working backwards:

On Thursday, I had £1.25.

On Wednesday, I had £1.25 x (1 ÷ one half) = £2.50.

On Tuesday, I had £2.50 x (1 ÷ two thirds) = £3.75.

On Monday, I had £3.75 x (1 ÷ three quarters) = £5.00.

Workings

Hint
If we look at all of the clues, each person appears exactly twice.

Answer
Alex had been teaching for 21 years.
Billie had been teaching for 15 years.
Charlie had been teaching for 7 years.

Reasoning
If we look at all of the clues, each person appears exactly twice.

So the total of the three clues gives us two lots of Alex + Billie + Charlie = 36 + 22 + 28 = 86.

So, Alex + Billie + Charlie = 43 years.

In each clue we are given two of the people, so we can use this to find the missing person:

   Charlie and Billie had been teaching for a total of 22 years, which means that Alex must be: 43 - 22 = 21 years.

   Charlie and Alex had been teaching for a total of 28 years, which means that Billie must be: 43 - 28 = 15 years.

   Alex and Billie had been teaching for a total of 36 years, which means that Charlie must be: 43 - 36 = 7 years.

Reasoning With Algebra
Let Alex = A, Billie = B and Charlie = C, then:

[1]   A + B = 36
[2]   C + B = 22
[3]   C + A = 28

If we use [3] - [2] we have:

[4]   A - B = 6

If we use [1] + [4] we have:

   2A = 42
    A = 21

By [1] B = 15. By [3] C = 7.

Workings

Hint
The answer isn't a whole number of years, and algebra might be required.

Answer
29 years 2 months.

Reasoning #1
This answer is taken directly from the original book.

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.

Reasoning #2
Here's my answer, with a little algebra.

If we call Tommy T, Mamma M and Papa P we can see that:

"our three ages add up to exactly seventy years" gives us:

(1)   T + M + P = 70

"Just six times as old as you" gives us:

(2)   P = 6 x T

In an unknown number of years (Y) "Shall I ever be half as old as you" gives us:

(3)   P + Y = 2 x (T + Y)

and "our three ages will add up to exactly twice as much as today" gives us:

   (T + Y) + (M + Y) + (P + Y) = 140

which can be written as

(4)   T + M + P + 3Y = 140

We can see from (4) and (1) that

     3Y = 70

so

(5)   Y = 70 ÷ 3

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

(6)             T = 70 ÷ 12

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.

Workings

Hint
Can the fractions be simplified?

Answer
667.

Reasoning
3      8      1
—  of  —  of  —  of 2001
4      9      2

which simplifies to:
24
——  of 2001
72

which simplifies to:
1
—  of 2001
3

Giving 667.