Topic, (GAME!!! :D) Brain teaser thing forum topic object...

  If you had an instrument that measured to an infinite accuracy, its not possible.
But as no such instrument exists, you can achieve it.

Assuming you used your car's clock to measure the time, there's is an accuracy of +/- 3 seconds throughout the day. So within an hour, you have an accuracy of +/- .125 seconds, or +/- .0000347 hours.

which means if the total time it takes you is 1.0000347 hours, as measured by your clock, that could be exactly an hour as far as you can tell.
This means if you traveled the remaining half distance in less then .0000347 hours you have achieved an average of 100 mph

To do this, your remaining speed must be slightly over 1,440,922 mph.

  Higher speeds than 300 000 km/s is impossible anyways. At high speeds time slows down. At low speeds it slows down infinitesimally. (at the speed of light it becomes difficult to talk about time even going)

This is all for the sake of the argument. Making an scenario to circumvent the actual solution (that there is no possible speed)is for fun.

  It would be possible if the time required was some infinitesimal amount. But it is zero exactly. Thus, even according to some theories, travelling with the speed of light slows down the time, it won't help. You need to teleport and nothing else works. If you try to solve the problem with some maths, you will need to devide by zero to find the speed. But since dividing finite number with infinitesimal number results in infinity, dividing finite number with zero results in number bigger than infinity (here we need to define "infinity" and some other things but I will not do it). So infinitelly big speed does not work.

  @GIS: If the watch measuring your time is placed in the car, it is possible... in theory, with enough assumptions ;) .

@Cubix: Miles, meters, yard are all measures of lenght: i.e. if one doesn't matter, the same applies to the others.

  It indeed is, Simon! :D

  @dingdong That's MILES. Not meters, MILES.

Jeez. Welcome to America.

  That's a pretty interesting thing!

  As you may have guessed, it needs to be exactly 100.
But that's not possible.
Say if your destination is 100 miles away.
You spend the first hour driving half the distance (50mph)
Your hour is over.
You can't drive 100mph anymore because you've already used up your hour.

  @Gis- it depends on the accuracy you want.
If you travel the remaining half distance at 50000 miles per hour, your average would be about 99.9 mph
If you travel even faster, you will get even closer to 100mph, but never reach that point exactly.

  @Cubix, twice as long as the distance you have driven when you change speed. What it is in meters doesn't really matter.

  What's the solution, gundu? I neither managed to guess it. (and couldn't find it via Google xD)

  @GIS, I googled that one, no way I would have guessed it

  Uh... how long IS the course?

Plus, Epic, I solved this one before. I know the answer.

 

  GIS: I would not recommend moving at the required speed if you're trying to lose weight ;) (I suppose the time is measured from the car)

  Here's one Elizea told me:

You car drives 50mph.
You're halfway there now, and you decide to change your speed so that you drove an average of 100mph once you reach your destination.
How fast do you need to drive for the other half of the course.

  ...
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connect all 9 dots using only 4 lines, the lines are allowed to overlap.

  um...

Yeah, I decided to make a topic for brainteasers. You can suggest a brainteaser, and I will sometimes post a brainteaser for you to figure out. You can post my inbox if you want to check the answer, and if you got one right, I will put a message saying "[USER] got [TEASER NO.] right!" Or something like that. If enough people get it right, you can post an answer on the topic itself, if it is in a "nested" spoiler (like... )
This goes for all other brainteasers. You, too, would post a teaser, expect a bunch of answers, post who got it. I'll start.

Which is worth more, pennies from the present, or 1987 pennies?

Got it? Remember, no peeking at an answer unless you are truly stumped.