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Question from Kenneth:


Here is my question:

Four carpenters can build eight houses in 10 days.
Two carpenters can build how many houses in 15 days?

I have determined the solution to the above by using the following calculation;

(2 X 15 X 8)/(4 X 10) that is divide (2 X 15 X 8) by (4 X 10)

Answer: 6 houses

Can someone explain why or how this solution will provide the answer?
What is the mathematical reasoning for this calculation providing the missing

I thank you for your reply.

Hi Kenneth,

First let me show you how I would solve this problem and then try to explain why the "formula you used" gives the same solution.

4 carpenters can build 8 houses in 10 days.

If you have half as many carpenters they can only build half as many houses in 10 days. Hence

2 carpenters can build 4 houses in 10 days.

If these 2 carpenters only have 5 days they can only build half as many houses. Thus

2 carpenters can build 2 houses in 5 days.

But if they have 15 days then can build 3 times as many houses so

2 carpenters can build 6 houses in 15 days.

Look at the number of houses in the 4 statements. The number of houses started at 8, then I multiplies by $\large \frac12.$ I then multiplied by $\large \frac12$ again and finally I multiplies by $3.$ Thus my calculation was

\[8 \times \frac12 \times \frac12 \times3.\]

But $\large \frac24 = \frac12$ and $\large \frac{15}{10} = \frac12 \times 3.$ so my calculation can be written

\[8 \times \frac24 \times \frac{15}{10} = \frac{8 \times 2 \times 15}{4 \times 10}\]


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