You may also like

Days and Dates

Investigate how you can work out what day of the week your birthday will be on next year, and the year after...

Natural Sum

The picture illustrates the sum 1 + 2 + 3 + 4 = (4 x 5)/2. Prove the general formula for the sum of the first n natural numbers and the formula for the sum of the cubes of the first n natural numbers.

Series Sums

Let S1 = 1 , S2 = 2 + 3, S3 = 4 + 5 + 6 ,........ Calculate S17.

Charlie's Delightful Machine

Age 11 to 16
Challenge Level

Why do this problem?

Many standard questions give exactly the information required to solve them. In this problem, students are encouraged to be curious, to go in search of the information they require, and to work in a systematic way in order to make sense of the results they gather.

The problem could be used to reinforce work on recording and describing linear and quadratic sequences.

Possible approach

This task will require students to have access to computers. If this is not possible, Four Coloured Lights provides students the opportunity to make sense of numerical rules without the need for computers.


Shifting Times Tables is a problem about linear sequences that could be used to prepare students for the thinking required in this problem.


Begin the lesson by dividing the board into two columns, one headed with a tick and the other headed with a cross.
Ask students to suggest numbers, and write each suggestion in the appropriate column according to a rule of your own choice. 


Here are some suggestions for rules:


  • Odd numbers
  • Numbers which are 1 more than multiples of 4
  • Numbers which are 2 less than multiples of  5
  • Numbers which are 3 more than multiples of 7
Once the class have tried the activity with a couple of rules, move on to the main task.


To introduce the main task (Level 1), show the interactivity and demonstrate entering a couple of numbers to see what lights up. Make sure students understand that more than one light can light up at once, and that each light is governed by its own rule.

Level 1 rules are linear sequences of the form $an+b$, with a and b between 2 and 12.

Students could then work in pairs at a computer, trying to light up each of the lights. Challenge them to develop an efficient strategy for working out the rules controlling each light.


While the class is working, note any particularly good ways of recording or working systematically, and highlight them to the rest of the class.

Bring the class together to share insights and conclusions before moving on to A Little Light Thinking, in which students are invited to find sequences that turn several Level 1 lights on simultaneously.

Key questions

Which numbers will you try first?

Which numbers will you try next?
How will you record your findings?
How many lightings are necessary to work out the rule for a light?

Can you suggest a number bigger than 1000 that you think will turn on the light?

Possible support

Shifting Times Tables offers students a way of thinking about linear sequences and opportunities to explore how they work.

Students could use a 100 square to record which lights turn on for each number they try.

Possible extension

Level 2 rules are quadratic sequences of the form $an^2+bn+c$ with a=0 or 1

Level 3 rules are quadratic sequences of the form $an^2+bn+c$ with a=0, 0.5, 1, 2 or 3.

Level 3 sequences can be used as a starting point for some detailed exploration into graphical representations of quadratic functions.