The subtle logic of this problem places it at this level.
Students are given three pieces of information which must be synthesised in order to find a solution. They must be attuned to the semantics of one piece of information, and find the right combination of factors for a given product, while looking for addends of a given sum.
Jack says, “Did you know that today is my three sons’ birthday?”
“How old are they?” asks Ollie.
Jack gives him a hint. "The product of their ages is 36 and the sum of their ages is 13.”
“That’s no help,” says Ollie.
Jack gives him another clue. “O.K. My youngest son is very naughty.”
“Nothing to it,” exclaims Ollie, and he tells Jack the correct ages of his sons.
How does Ollie figure out the correct answer and what are Jack’s sons’ ages?
- Talk about the problem with the class. See if they have any ideas. Ask:
Can you summarize the problem in your own words?
What are the important ideas in this problem?
What strategies might you be able to use?
Do you think it is useful to make a table? If so, how?
- As they work in their groups you might ask:
What do you understand from the statement “All my three children are celebrating their birthday today”?
Do you think the last hint is very important? In what way?
How many possible answers are there that satisfy the first two hints?
- As groups solve the problem, encourage them to record their solutions as they work. Also ask:
Have you considered all the cases?
Have you checked your solutions?
Does it look reasonable?
Are their any other solutions?
Are there any shortcuts other than making a table?
- Have students share their solutions before trying the extension.
- the product of the ages is 36;
- the sum of the ages is 13;
- the youngest of Jack’s sons is very naughty.
Sum of their ages
From the table we can see that there are two lots of ages (factors of 36) that add up to 13. These are 9, 2, 2 and 6, 6, 1.
It’s not surprising that there are two answers. If there was only one, Ray would have been able to solve the problem without any further clues. But what possible help can it be to know that Jack’s youngest son is very naughty?
The point here is that Jack has a youngest son! He doesn’t have two sons that are the same age. So 9, 2, 2 can’t be the answer. The answer has to be 6, 6, 1, where there is a youngest son whose age is 1.
Ray correctly identified the ages of Jack’s sons as 6 years, 6 years and 1 year.