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11
Apr 07
Wed

Maths problem

Two minutes to solve, no calculators:

My brain’s not working – I’ve hit a mental blank with this – answer in the comments section please.

  11:31pm (GMT +10.00)  •  Miscellaneous  •   •  Tweet This  •  Comments (6)

This post has 6 comments

1.  fjord   Apr 12 2007 • 1:37 am

The first one. Er… the proof takes more than 2 minutes for ordinary people… You need to prove (among other things) that n is divisible by 30.
2.  fjord   Apr 12 2007 • 1:45 am

Er… On second thoughts – You don’t really need to prove n is divisible by 30.
3.  Shishberg   Apr 12 2007 • 8:03 am

First one it is. Spoiler anyone?
4.  Stu   Apr 12 2007 • 8:16 pm

Yeah – I know the answer – but does anyone have a 2-minute method?

BTW, the question is phrased such that expressions I, II and III may all be correct (ie, there may be more than one correct answer), so keep that in mind regarding the method…
5.  Shishberg   Apr 12 2007 • 10:48 pm

The thing to realise is that since 450y is a cubed number, its prime factors are in triples.

450 is 2 * 3*3 * 5*5, so y needs to have factors of at least 2*2 * 3 * 5, i.e. answer I.

Didn’t time how long it took to get that, but I’d be surprised if it was more than a minute.
6.  yennus   Apr 14 2007 • 5:51 pm

I took over 5minutes and still couldnt do it…

Woof :(

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