Three classes are collecting cans for the Thanksgiving food drive. Class A collected 275 cans. Class B collected 225 cans. Class C collected 253 cans. These cans need to be put in boxes of 24 before they can be delivered to the local food bank. How many boxes are needed?
We chose the numbers deliberately. First we wanted to see who would look at the numbers and add them mentally and who who would automatically put them in columns to add. Then we wanted to see what students would do with the remainder. We were also interested in seeing how students would do the division.
As it turned out, all but one pair solved the problem correctly but they did so in varying ways. Yes, some added the numbers in a traditional way which is fair enough. Others though said "I know 25 and 75 is 100 so I added this to the other 400 to get 500 and then added the 253 to get 753," or a variant of that. Most students thought this was neat and it allowed us to give this bit of advice:
Good mathematicians look at the numbers first before they decide what strategy to use.
So, 506+499... do that in my head. 689+7259+164... maybe do that on paper.
With the division, some used a traditional long division approach. Others used an abbreviated version of this (I think this is called short division).
Other students used a chunking method using friendly numbers (this is what I tend to do if I do a division in my head).
So in our Consolidation, we asked the students this very question. They were pretty much in agreement that it meant numbers you could work with easily in your head. We also had students comment that a friendly number for you might not be friendly for someone else. With this in mind, I asked the group who started with a chunk of 9 the following question:
If you could go back in time and give yourself a bit of advice before you start this question, what would it be?
Straight away, they replied "Don't use a group of 9. Use a group of 10 instead as it is way easier."
Other groups, having seen the chunk of 20 approach (above) said that they wouldn't just stick to chunks of ten and would look to use 'larger chunks of 10' i.e. multiples of ten.
So there was some really good thinking and understanding going on. With some well chosen practice questions, this understanding will continue to be solidified.