Why do this problem?

This problem encourages students to think about the properties of numbers, including divisibility and remainders. It may help students gain a deeper understanding of linear sequences (and perhaps quadratic sequences if they explore Level 2 and 3). The interactivity encourages students to begin the problem experimentally before working more theoretically as they engage with the main ideas.

### Possible approach

*This printable worksheet may be useful:* LightThinking.pdf
This problem follows on from

Charlie's Delightful Machine, so we assume students have a strategy for determining the rules to switch individual

**Level 1** lights on.

Here are some questions you may wish to ask, to prepare students for what comes next:

*What is special about a ***Level 1** rule where all the 'light on' numbers
*are odd?*
*are even?*
*are a mixture of odd and even?*
*are all multiples of 3? Or 4? Or...*
*have a last digit of 7?*

"Did anyone find examples where two or more (level 1) lights switched on simultaneously?"

**"Our challenge now is to determine how to turn on two (level 1) lights (or more) simultaneously, if we know the rules for each individual light."**
If computers are available, students could work in pairs determining the rules for individual lights, then finding the rule for switching on pairs of lights simultaneously. Remind them of the importance of recording their findings to share with the class.

If computers are not available, students could explore pairs of linear sequences, searching for examples where two sequences have numbers in common.

One way they could record their work is by creating a table with sequence rules along the top and down the side, and indicating with a tick or a cross in each cell whether both lights could light up simultaneously. When appropriate, they could also indicate numbers which successfully switch on both lights.

Remind the class that the task is not simply to find examples, but to find a way of determining, by just looking at the sequence rules, whether or not a pair of lights will ever light up simultaneously.

When trying to produce convincing arguments, ideas from the problem

Stars might be useful.

Leave some time for the class to come together to share examples of rules where more than one light could be switched on simultaneously, and examples where it was impossible, together with their reasoning.

### Key questions

What is true about any pair of rules where it is not possible to light up both lights?

If two sequences are described by the rules $an+b$ and $cn+d$, can you explain the conditions for determining whether the lights will ever switch on together?

### Possible extension

Some students may wish to use ideas of modular arithmetic to prove their findings; reading this article first may help.

### Possible support

The problem Shifting Times Tables offers an introductory challenge for exploring linear sequences.

The problem Remainders explores some properties of numbers which could be useful when thinking about this problem.

Students could use a 100 square as a visual way to record sequences and see where (if) they coincide.