This primary school kid's maths homework has left mums baffled… so can YOU solve it?

One woman took to Mumsnet to share her youngest daughter's lighthouse related homework question – explaining that she's in Key Stage 2, or school years 3-6.


Lucysmam begged her fellow parents: "I'm baffled by it even though, since it's dd2's homework, I probably should be able to work it out. Someone explain…please?"

An equally confused AjasLipstick replied: "That is a ridiculous question and I'm only here to learn something.

"I'll wait till' someone better than I am comes along."

And Beaverhausen added: "Bleurgh my dd is also in ks2 and sometimes I just googled the homework lol".

Here's the full question

On the coast there are 3 lighthouses.

The first light shines for 3 seconds, then is off for 3 seconds.

The second light shines for 4 seconds, then is off for 4 seconds.

The third light shines for 5 seconds, then is off for 5 seconds.

All three lights have just come on together.

1) When is the first time all three lights will be off together?

2) When is the next time all three lights will come on at the same moment?

Most of the parents agreed on the answer to the first question, but were divided on the second one – with some mums thinking the answer was 25, 120 or even the first 3 seconds.

Still feeling baffled? We'll put you out of your misery. The first time they will all be off is after 5 seconds – and the answer to the second question is 120.

To work out the next time all three lights will all come on together, you have to work out the lowest common denominator of 6, 8 and 10 – which is 120 secs.

As one mum explained: "The first light comes on every 6 seconds, the second every 8, the third every 10.

"So what the second part of the question is asking is: what is the lowest number (other than 0) that is divisible by 6, 8 and 10? (It’s 120 btw)."

Did you crack it? Don't feel too silly if not, the the mums had to Google the answer to settle the debate once and for all.

A "super simple" maths puzzle about a horse previously sent mums into a similar frenzy, so can YOU crack it?

And, if you're in a quizzical mood, take these genuine questions – to see if you could pass your SATs.

Or see if you can work out which of these copycat looks used 21 make-up products and which used six?

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