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fuse on fire: variation

Here's a variation on the "fuse on fire" problem:

You have a torch and two fuses (pieces of string) of varying thickness which each burn for 40 seconds. You want to measure 30 seconds. You can't cut the one fuse in half because the fuses are different thicknesses and you can't be sure how long each will burn.

How do you know when 30 seconds have ellapsed?

Christoph Sommer
Wednesday, June 01, 2005

Folding the strings?

Fold one in half; fold one in quarters.
Burn the half-fold through.
Then light the quarter-fold and burn it through.

Yeah, right, unless I'm right...
Thursday, June 02, 2005

Easier Way:

Light both ends of one string (either one).
It'll burn down in 20 seconds no matter how irregular.

At the same instant light one end of the second string.
It'll also burn off some while the other is totally consumed.
In 20 seconds, when the first string is consumed, light the second end of the second string.
This will burn off what's left in 10 more seconds.

Yeah, right, unless I'm right...
Thursday, June 02, 2005

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