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Magic Plant

Age 5 to 7
Challenge Level

Lots of you sent in your solutions to this problem - unfortunately far too many to mention everyone. Most of you worked out the plant's height each day, which is a very good way of approaching the problem. Here is an example of this method from Tom who goes to Linden Primary School:

So first you start with 2 centimetres.
On Saturday it's double 2. And double 2 is 4.
On Sunday is's double 4. And double 4 is 8.
On Monday it's double 8. And double 8 is 16.
So the answer is ...16 centimetres.

This is very clearly recorded - well done Tom. Hannah, Grace and Ellie, who were attending Royal Institution masterclasses, sent a very similar solution. So did Oliver from Webbers School, Livvy (who didn't say which school she is from) and someone from St Joseph's School in Harrogate. Ha Young from Wesley College wrote down the same calculations in a slightly different way, showing that doubling means multiplying by two:

On Friday at 9am, it was 2cm
On Saturday at 9am, it was 2x2=4cm
On Sunday at 9am, it was 2x2x2=8cm
On Monday at 9am, it was 2x2x2x2=16cm
So, the answer is 16cm.

Some pupils from Queens Crescent School also wrote down something a little different. Rather than recording multiplication, they representated the calculations as addition, like this:


In other words, doubling a number, or multiplying it by two, is the same as adding it to itself. Very well done all of you.