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# Flashing Lights

*(Thank you to Norrie McKay and the lights over Tokyo Bay for
this problem)*

2nd light 0 - 5 - 10 - 15 - 20 - 25 - ........

They flash at the same time every 20 seconds 0 - 20 - 40 - 60

That's four times in all.

For two lights the pattern was every 20 seconds and 4 x 5 = 20

For the three lights it is going to be 4 x 5 x 7 = 140 seconds or 2 minutes 20 seconds

You need to find a multiple of 20 and seven to work out how many minutes before all of the lights flash together. 20 x 7 = 140 sec = 2 mins 20 sec

Well done to all of the following people: Jesse, Edward, Daniel and Thomas from Tattingstone School who did some very good work with finding the multiples. Lily from Sotogrande International School, Daniel from Anglo-Chinese School, Singapore, Abigail, Charles and David from Moorgate Primary School, Staffordshire, Jason from Priory School, Thomas from St Francis School, Maldon and Ashley.

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Age 7 to 11

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There were many correct answers sent in
for this problem. As **Primary Maths Club** (International School of Toulouse) pointed out, it
helps if you start counting seconds from the first time the two
lights flashed together (at zero seconds).

Some people thought about a number line, others looked for a number that both of the numbers of seconds (4 and 5) would divide into (common multiple). Here are two very well explained solutions.

**Holly,
Harriette, Caroline, Florence** and
Rebecca from The Mount School, York:

2nd light 0 - 5 - 10 - 15 - 20 - 25 - ........

They flash at the same time every 20 seconds 0 - 20 - 40 - 60

That's four times in all.

For two lights the pattern was every 20 seconds and 4 x 5 = 20

For the three lights it is going to be 4 x 5 x 7 = 140 seconds or 2 minutes 20 seconds

Christina from Marlborough Primary School :

To work this out you need to find a multiple of both 5 and 4 which is 20. So the lights flash together every 20 seconds and to find out how many times they flash in one minute you need to do 60/20 = 3 which means that they flash together 3 times a minute.You need to find a multiple of 20 and seven to work out how many minutes before all of the lights flash together. 20 x 7 = 140 sec = 2 mins 20 sec

Well done to all of the following people: Jesse, Edward, Daniel and Thomas from Tattingstone School who did some very good work with finding the multiples. Lily from Sotogrande International School, Daniel from Anglo-Chinese School, Singapore, Abigail, Charles and David from Moorgate Primary School, Staffordshire, Jason from Priory School, Thomas from St Francis School, Maldon and Ashley.