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Making 9

Published 12 months ago by Johnny Baker


One can make challenging problem out of the simplest of materials.

Take the numbers 1 to 8. Thinking easy? Well, teachers in our workshops might well disagree!

We ask them to make two numbers whose total is 9 and which use the digits 1, 2, 3, 4, 5, 6, 7 and 8 once only.

Even for adults, this problem generates a lot of useful discussion at the S stage of the STAR problem solving model (Sort it out).

STAR Problem Solving

Have you heard about the Manu Kapur concept of ‘productive failure’. The experiences of the teachers was of the ‘productive failure’ type, because as they struggled with sorting out what the question meant, they had to cycle through a number of concepts and clarify what was involved in this problem.

We heard questions such as:

These questions were answered through discussion and discarding ideas that did not fit the consensus views. For example, a digit isn’t a number, it is one of the symbols that we use to make a number. That is, 5 and 7 are digits but 75 and 557 are numbers that can be made with those digits. A total is made when two or more numbers are added and does not refer to the outcome of a subtraction or multiplication.

A scaffold that might help you think about this problem:

make-9-scaffold

IF YOU'RE GOING TO HAVE A GO... Read no further for now ...

Hint:

The initial failure to find the right meaning for these terms eventually gave way to exploring how two numbers could be made with the digits 1 – 8 to reach a total of 9.

These ideas were the key that unlocked the solutions.

Soon it was realised that two 4-digit numbers were needed and for their total to be equal to 9 the decimal fraction parts would have to reduce to zero. For this to happen, the thousandths have to add to 10. The tenths and hundredths have to add to 9 and the units have to add to 8 as shown below.

make-9-scaffold2

And of course - there is always a Sting in the Tail at Natural Maths! Here's a solution ... are there more?

make-9-scaffold3

Would you like to be able to challenge your students with great problems? Check out our Problematised Situations online course here.

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