Idea 216: ‘Number Elimination’. Problem Solving Lesson With Place Value, Activity for All Primary Ages


Sometimes (rarely!) I get a bit of a creative spark for a lesson which actually works out, and that’s exactly what happened here!

I made grids of numbers for my children along with a list of instructions for how to go about eliminating numbers. The children have to work through the steps, crossing out numbers as they go until they are left with the one ‘secret’ number.

My lesson went like this:

I introduced the activity and we worked through an example as a class, with the children discussing each step with their partner and eliminating numbers on their mini-whiteboards.

Next, the children could choose which of three levels of challenge they would like to take on. I put the sheets up on the smartboard so they could see them and get a feel for the level of difficulty each had. I made these sheets in probably twenty minutes:

Level 1 challenge


Level 2 challenge


Level 3 challenge


Usually I have a lot of collaborative learning, but this lesson I wanted to work on resilience, so I had the children work in silence independently on trying to crack the secret number. They loved it!

When children think they have finished they can come and show you. You can pretty easily see if they have the correct secret number and give them some feedback if not such as: ‘it says to cross out any even number, so how could the secret number be 84? Have another try!’.

TIP: If the first person who gets the answer blabs, then you’re in trouble! I made this lesson a competition: all sheets would go into a pile at the end of the lesson and I would draw a few sheets out at random. If that sheet had the correct answer, we’d do something nice for that pupil (a big cheer, team point or whatever you like!). The more difficult the challenge that was completed, the bigger the cheer!

HOWEVER, I made a big point that if I heard anyone going round telling other people the answer, both they and the person they told were DISQUALIFIED. This made a big difference I think as I had no issues at all in the lesson with children telling each other the answer.

TIP 2: Make sure you have a lot of extra copies of the sheets! That way when a child comes to you and shows you their sheet, if they have the wrong answer they can quickly start again.

TIP 3: In the shared example that you do together, show how you can cross out a number with one diagonal dash, then at the end you can start back and check with another diagonal dash to make a cross over each number. This models a nice easy way for the children to start again on the same piece of paper if they make a mistake.

DIFFERENTIATION: You can change the size of the numbers and difficulty of the questions easily to adjust this activity for pretty much any achievement level or age.

EXTENSION: If children finish a grid, they can move up a level. If they complete the top level of challenge they can have a try making their own grid to test their friends with.

This entry was posted in Mathematics and tagged , , , , , , , , , , , , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *