What Numbers Can We Make Now?

 

This problem should be attempted after working on What Numbers Can We Make?

 

Start by showing the interactivity from the problem, and clicking on 'New Numbers' several times:

"This interactivity can generate lots of different sets of bags like the set we worked on last lesson. Later on I'm going to generate a set of bags and ask you what is special about the total when I choose three, four, five, six... 99 or 100 numbers. To prepare a strategy for answering these questions, here are some bags to get you started."

 

Display this image (available as a PowerPoint). Then arrange the class in pairs or small groups, and allocate one or two sets of bags to each.
 

 

While groups are working, circulate and listen for any useful insights to bring out in the whole class discussion later. If anyone finishes their set of bags early, they can apply their strategy to someone else's set.

 

Bring the class together, and invite groups to share what they found. Then allow groups a few minutes to discuss a general strategy for answering the questions generated by the interactivity.

 

Finally, display the interactivity again. Generate new questions, and invite the groups to use their strategy to work out what happens for three, four, five, six, 99 or 100 numbers. Check their answers, and then repeat, giving each group a chance to have a go at answering a 99 or 100 question.

 

You could finish off by asking the final question from the problem:

"If the bags contained 3s, 7s, 11s and 15s, can you describe a quick way to check whether it is possible to choose 30 numbers that will add up to 412?"

 

 

If I choose 5 numbers that are each one more than a multiple of 5, what is special about their total? Why?

 

Possible support

Begin by asking students to explore what happens when they add numbers chosen from a set of bags containing 2s, 4s, 6s and 8s.

They could then consider what happens when they add numbers chosen from a set of bags containing 1s, 11s, 21s and 31s.

Can they explain their findings?