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NRICH: A Little Light Thinking

Here is a machine with four coloured lights. Can you make two lights switch on at once? Three lights? All four lights?

Can you figure out a method to make all four lights on the machine below switch on at once?

This problem follows on from Charlie’s Delightful Machine, where you are invited to find efficient strategies for working out the rules controlling each light.

The rules for turning on the Level 1 lights are all given by linear sequences (like those found in Shifting Times Tables).

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?

Can you make two Level 1 lights light up together?

Once you have made two Level 1 lights light up together, can you find another number that will light them both up? And another? And another? …

Can you find any connections between the rules that light up each individual Level 1 light and the rule that lights up the pair?
What about trying to light up three lights at once? Or all four?

Sometimes it’s impossible to switch a pair of Level 1 lights on simultaneously.
How can you decide whether it is possible to switch a pair of lights on simultaneously?
Or a set of three lights? Or all four?

Age 14 to 16

Algebra & Pre-Algebra, Linear Equations, Patterning & Sequencing
High School, Educator

## Organization

NRICH (University of Cambridge)

Challenge