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What's Left?

Stage: 1 Challenge Level: Challenge Level:2 Challenge Level:2

What's Left?


Here is a grid showing the numbers $1-20$:

Numbers 1-20 arranged in numerical order in 2x10 grid with 1-10 on top row, 11-20 on bottom row. Numbers 2-5 are in red

Start with the first red number, $2$. Now, look along the top row of the grid to the next number, $3$.

Can you share $3$ exactly between $2$?

If the answer is yes then shade in the $3$ box. If the answer is no, leave it.

Now take the next number, $4$. Can you share $4$ exactly between $2$?

Again, if you can, shade the box with $4$ in it. Carry on doing the same with all the numbers.

Now do the same with the next red number, $3$. Look along the row for the next number, $4$ and shade all the numbers that will share exactly between $3$.

Now try with $4$ and with $5$.

Which numbers do you have left?


Why do this problem?

This problem will help young learners to come to some understanding of factors and multiples leading to a first introduction to the idea of prime numbers. They will need to look at multiples of $2$, $3$ and $5$ up to $20$. For some children just making the table to use will be a useful exercise.

Key questions

Do you know the name of the numbers you have coloured in now you have done the sharing by $2$?
Do you know the name of the numbers you have left out now you have done the sharing by $2$?
Which multiplication table will you find these numbers in?
Which numbers have not been coloured in at all?

Possible extension

Learners could make a grid which goes up to $40$ or even $100$ adding $6$ and $7$ to the "red" numbers.

Possible support

Suggest starting by making the table to use. Then colour in the numbers counting in $2$s omitting $2$ itself.