Mental Mondays #9

This is a great little Mental and Oral starter in which the children have to identify the different digits within a number.

The focus is on using number properties and problem solving strategies. However, this starter also reinforces place value.

Here's how it goes. You display four different digits (using digit cards, or by writing the numbers on the board), then you offer a series of clues as to the position of those digits in a 4-digit number.

If the children work in pairs with mini-whiteboards they will be able to quickly share their ideas and strategies. It may be useful to let them first drew a place value template:

Now, you display the number 1, 2, 3 and 4 (for example) and the children have to work out what the four digit number should be by using the following three ‘clues’:

• The units digit is double the tens digit
• The hundred’s digit is odd
• The thousands digit is 1 less than the units digit

These three clues can be presented all at once, or revealed one at a time to see how each one helps the children to re-think the problem. Let the children explore the possibilities and then encourage them to explain their thinking.

There is only one solution here: 3124

• The units digit is double the tens digit - 4 is double 2
• The hundred’s digit is odd - 1 is the hundred's digit
• The thousands digit is 1 less than the units digit - 3 is one less than 4

Here's another one:

Using the digits 3, 4, 6 and 8, the instructions are:

1. The units digit is half the size of the thousands digit
2. The tens digit is double the tens digit
3. The units digit is odd

Solution: 6483

You can add more challenge by increasing the number of digits and/or the complexity of the clues:

Using 1, 3, 4, 6 and 9

• The hundreds digit is even
• The thousands digit is an odd multiple of 3
• The ten-thousands digit is the sum of the tens and the units digits

Solution: 49631

An alternative way of increasing the level of challenge would be to include one unknown number:

Using 3, 7, 9 and ? (where the unknown digit is a triangular number)

• The hundreds digit is a prime
• The units digit is odd
• The Thousands digit is double the tens digit

Solution: 6739 - the unknown number was '6'

You can follow this activity up by challenging the children to develop their problems.

Finally, you can easily incorporate this activity into a range of other contexts. We brought in a digital cable lock and suggested to the children that the 4-digit number was the code to the lock. We said we had forgotten the code, but could remember a few facts about it. The children then were eager to find the number.

When all the solutions were in we chose one group to put the number into the cable lock to see if they were right.

This is a great paired mental and oral starter. We hope you and your children enjoy it as much as we did.

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