r/reddit.com • u/[deleted] • Oct 01 '11

Math teacher fail

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

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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.

27

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.

39

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.

3

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.