Jump to content

Can you solve the maths question for Singapore schoolkids that went viral?


Recommended Posts

That's like the one where you are given the following instructions:

 

Start a stop watch.

 

Go and switch you bedroom light on for one hour.

 

Then go and switch it off for half an hour.

 

Then go and switch it on for fifteen minutes

 

Then go and switch it off for 7 and a half minutes.

 

Keep doing the same routine.

 

Is your bedroom light switch on or off after 2 and a half hours?

Link to post
Share on other sites
  • Replies 287
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Popular Posts

Sorry chinahand. Had visitors, will ring you back shortly.

Looks to me like chinahands kitchen after he finished tiling it

Can you solve the maths question for Singapore schoolkids that went viral?

Posted Images

Impossible to say on those instructions China - the series converges on 2 hours. I suspect the switch will however be neither on nor off but broken. Unless of course you mean to repeat those 4 instructions rather than follow the ever decreasing pattern, in which case after 2 and a half hours it'll be on.

Link to post
Share on other sites

Oh what the heck one last maths question on a Friday Night!

 

You are planning to drive across the Golden Gate Bridge turn round and come back - 3 miles from start to finish.

 

And being a bit of a neurotic you want to do it at exactly the speed limit 55mph.

 

Sadly you get held up and after a mile you've only averaged 18.3333 mph. On average how fast must you drive the remaining 2 miles to achieve your wish?

  • Like 1
Link to post
Share on other sites

Oh what the heck one last maths question on a Friday Night!

 

You are planning to drive across the Golden Gate Bridge turn round and come back - 3 miles from start to finish.

 

And being a bit of a neurotic you want to do it at exactly the speed limit 55mph.

 

Sadly you get held up and after a mile you've only averaged 18.3333 mph. On average how fast must you drive the remaining 2 miles to achieve your wish?

To travel 3 miles at 55mph (miles / hour) would take you; 55 = 3/h so 55h = 3 so 3/55= h = 0.05454 hours.

 

Travelling 1 mile at 18.333 mph would take you 18.3 = 1/h so 1/18.333 = h = 0.05454 hours

 

So Wrighty is right.

  • Like 1
Link to post
Share on other sites

Yep, it's a trick question.

 

Usually if you try to solve it using standard maths techniques for average speeds you end up trying to divide by zero.

 

The point of the question is to get you to think why it is making you divide by zero - you haven't made a mistake, the question is requiring you to instantaneously get to the finish point - something that requires either infinite speed or stopping time - hence c is the right answer!

 

Your explanation gets that point perfectly.

  • Like 1
Link to post
Share on other sites

When averaging speeds you have to use the harmonic mean. For example travel a mile at 60mph and then a mile at 30mph - what's your average speed? Not 45mph, which is the obvious answer, but 1/(0.5x(1/60 + 1/30)) - average the reciprocals and then reciprocate. 1 mile at 60mph takes 1 minute, 1 mile at 30mph takes 2 minutes, so the average speed is 2 miles in 3 minutes, or 40mph.

 

This question is basically this - what speed do you have to travel to solve 55=1/(1/3x(2/? + 1/18.3333)) (it's a weighted harmonic mean) and the ? comes out as infinite. Speed of light is as close as you can get, and taking into account special relativity, it is basically infinite - but that's for another thread.

  • Like 2
Link to post
Share on other sites
  • 1 month later...

Another Math's question confusing the heck out of GCSE students:

 

'There are n sweets in a bag. Six of the sweets are orange. The rest of the sweets are yellow.

'Hannah takes a sweet from the bag. She eats the sweet. Hannah then takes at random another sweet from the bag. She eats the sweet.

'The probability that Hannah eats two orange sweets is 1/3. Show that n²-n-90=0'

Link

Link to post
Share on other sites

Too easy! The comments on that link are laughable - 4 accountants taking 2 hours to solve it. Don't think they'll be doing my tax return.

 

I'll try my kids with it later - one's just done GCSE but a different exam board, the other got an A* GCSE last year and has just done AS. I'll be disappointed if it takes them much longer than the few moments it took me to solve.

Link to post
Share on other sites

Another Math's question confusing the heck out of GCSE students:

 

'There are n sweets in a bag. Six of the sweets are orange. The rest of the sweets are yellow.

'Hannah takes a sweet from the bag. She eats the sweet. Hannah then takes at random another sweet from the bag. She eats the sweet.

'The probability that Hannah eats two orange sweets is 1/3. Show that n²-n-90=0'

Link

Yeah, that's a piece of piss, and I think the problems just come from not reading the question properly. Hanna getting 2 orange sweets is 1/3, so

 

1/3 = 6 out of n chance it'll be orange * 5 out of n - 1 chance it'll be orange

1/3 = 6/n * 5/n-1

 

Re-arrange it

 

1/3 = (6*5) / n(n - 1)

1/3 = 30 / n squared - n

1 = 90 / n squared - n

90 = n squared - n

 

0 = n squared - n - 90

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
  • Recently Browsing   0 members

    No registered users viewing this page.


×
×
  • Create New...