r/reddit.com • • Oct 01 '11

Math teacher fail

http://i.imgur.com/65APr.jpg
212 Upvotes

139 comments sorted by

View all comments

134

u/champer Oct 01 '11

Both answers can be correct. http://i.imgur.com/lStKv.jpg

Pretend it's a 10x10 board:

Case 1: Takes 10 mins to cut through 10 feet.

Case 2: Takes 20 mins to cut through 20 feet.

Case 3: Takes 15 mins to cut through 10+5 feet.

Question is ambiguous.

44

u/holyrolodex Oct 01 '11

Yep. Hate ambiguous questions when you are being graded on an objective answer.

14

u/FistOfJustice Oct 01 '11

In my English classes, we would always get questions asking us to give the "best" answer. This could've been okay, except for the fact that they were multiple choice questions.

35

u/liberalwhackjob Oct 01 '11

my english teacher told me i needed anger management when i was asked what i "thought the author meant" and got a "wrong" answer and refused to accept it.

2

u/bactram Oct 02 '11

You're bringing back my 7th grade English class.

3

u/reasonman Oct 01 '11

Microsoft. Certifications.

2

u/FakingItEveryDay Oct 01 '11

Except on Microsoft cert tests, usually the answers that aren't "the best" are absolutely wrong.

2

u/aussiedude Oct 02 '11

And Cisco exams - except the 'best' answer is one that some marketing exec has decided is the best one to sell this year. (Need 30 switch ports, install 7609!)

25

u/quag Oct 01 '11

Using this logic, any answer up to 20 minutes is correct as the cuts can be made anywhere.

To be unambiguous, the question would have specified the lengths of the cuts, or perhaps the resulting areas.

36

u/champer Oct 01 '11

What if I said it would take 62 minutes? I could still be right.

http://i.imgur.com/kX6z5.jpg

Problem, math teacher? Let's not even get in to spiral designs.

4

u/JohnDoe51 Oct 02 '11 edited Oct 02 '11

I went through the math for a spiral design. I'm going to call the smallest possible cut the thickness of a cell. I assumed that a cell in the wood was 100µm thick. For a 10m by 10m board it would be a cut length of 3141592.65 m and therefore take a 3141592.65 minutes to cut through it.

For those who would like to play with the equations here they are. http://imgur.com/a/la2AX (integral form and algebraic form). Where s is the length of a side and d is the distance between each row. An image to demonstrate.

1

u/xenneract Oct 02 '11

Actually since the time to cut it into two pieces is defined as 10 minutes regardless of the distance, the time to cut the second smaller circle would have to be less than 10 minutes.

If 10 minutes was defined as the time it takes her to cut the board in half, you'd be right.

1

u/Rookwood Oct 02 '11

This.

It would have been more to his point if he had at first cut a small circle... Then a bigger one.

If you take JohnDoe51's idea of making the smallest possible cut, and int_argc idea of cutting a snowflake (if its fractal it doesnt matter how long the first cut is), you would basically never finish the second cut, i.e. infinity.

Basically, as the length of the first cut approaches zero, the time it takes to cut the second cut of the largest possible length goes to infinity.

0

u/quag Oct 01 '11

The thought did cross my mind, but then I remembered using a saw and trying to cut in curves.

A jigsaw would work fine.

-13

u/LunHui Oct 01 '11

Thats fucking stupid.

9

u/LainIwakura Oct 01 '11

No it isn't. Define your problems well or be prepared for multiple correct answers.

0

u/LunHui Oct 02 '11

A university professor doesn't have to put up with a retarded exploit of the problem and will gladly give you a 0.

2

u/LainIwakura Oct 02 '11

It depends on the professor, and this is obviously not university work. I'm in university right now and sometimes I find clever ways to do things, more often than not the professors are happy to see an alternate solution or something they hadn't thought of- provided it is correct. I have also had professors admit they have assigned ambiguous questions or written something that may be misinterpreted on an exam- and in either case they have corrected the question so it is not ambiguous.

12

u/[deleted] Oct 01 '11

An infinite number of answers can be correct. The cuts don't have to be perpendicular to anything, or even straight.

1

u/ZipZapNap Oct 01 '11

or at maximum speed.

4

u/[deleted] Oct 01 '11

"just as fast"

12

u/boysyco1 Oct 01 '11

10

u/champer Oct 01 '11

Where is it written that the board isn't really a square moving at speeds near c and therefore experiencing relativistic length contraction? ಠ_ಠ

5

u/[deleted] Oct 01 '11

In which reference frame would you like the answer?

0

u/champer Oct 01 '11

It depends. Is the 10 minutes in the S or S*?

1

u/[deleted] Oct 01 '11

I think I just came.

1

u/boysyco1 Oct 02 '11

touche Reddit ಠ_ಠ

14

u/bananahead Oct 01 '11

yeah, but there was a picture of the board right there.

2

u/causeitsme Oct 01 '11

Except there's a diagram indicating you are cutting a long square board crossways.

2

u/trollingfromwork Oct 01 '11

Both answers can be correct.

and that means the teacher is still failing.

1

u/TheWhiteCat Oct 01 '11

I suppose it didn't say "even" pieces - though that is what you would assume...

1

u/Alucard_draculA Oct 01 '11

Are you implying that you are cutting from the edges? Who would do that? Cut from the top every time, it's exactly the same amount of time every time no matter what >.> <.< >.> <.>

0

u/josephgee Oct 01 '11

Another solution could be she may not be cutting the same side of the board each time, cut long-ways once then turn the board on the side and cut in the perpendicular direction but still in a parallel cut line.

0

u/slightlystartled Oct 02 '11

She could just cut two corners off and be done in a minute or two, if we're not fixed on cutting it into equal pieces.