Okay, just to start off. (I leave all my ideas so others can watch and get new ideas on my failed attempts) 

If you simply did

> Sprinter + regular = 2 minutes

< Sprinter alone = 1 minute

> Sprinter + Fat Guy = 5 minutes

< Sprinter alone = 1 minute

> Sprinter + grandma = 10 minutes

So that's 19 minutes. We have to figure out a way to beat that.

So, here's an idea.

> Sprinter + Regular = 2 minutes

< Sprinter = 1 minute

> Fat guy + Grandma = 10 minutes

< Regular = 2 minutes

> Sprinter + Regular = 2 minutes

That's 17 minutes.  

> Sprinter + Regular = 2 minutes

< Regular = 2 minutes

> Fat + Grandma = 10 minutes

< Sprinter = 1 minute

> Regular + Sprinter = 2 minutes

That's really the same thing, and also gives 17 minutes.

A guy lives in a building of 50 floors. When he arrives at the building at the end of the day he gets into the lift and goes up to the 26 floor and gets out and climbs the stairs the rest of the way. Why does he do it?

Ask any question that i can answer yes or no to.

Because he is too short to reach the button higher than 26th floor, as the buttons on the elevator go like this. 

1 --- 26
2 ---- 27
... ----...
24 --- 49
25 --- 50

Well done

Ok another one

Two customers are in a bank when two bank robbers rush in and rob the bank. While they are in the bank the cops turn up and the bank robbers take the customers hostage. They take them to their getaway car and put both in the boot(trunk). They drive off and drive for a little while and then stop and open the boot and find 1 customer is dead and the other is fine. Why?

Hey, that was it? Awesome.

Okay, I have a math one that's probably really easy, but if you think the wrong way, it may not be that easy. I'm not even sure it's a brain teaser. 

Using the 2 digit system (binary), 3 digit system, and the 5 digit system, your mission is to get 210.

The catch though, is that you must use exactly 12 digits in total over the 3 systems. 

If someone has a better one, add it, and we'll do that one instead, as this one is probably a bit dull to most people.

First of all, you were right about the bridge question! Congratulations!

Now about your brainteaser, I have a question we're looking for a total of 210 in the 10 digit system right?

If that's the case you could go with :

1000 (binary) + 2121 (3 digit system) + 1000 (5 digit system)

15 + 70 + 125 = 210

Ok, so here's the one I promissed:

There are two integers, x and y, greater than 1, not equal and their sum is less than 100. One mathmatician has the sum of these numbers and another one has the product.
The guy that holds the product says: "I don't know what the numbers are."
The guy with the sum responds: "I knew that you wouldn't have the numbers."
The product guy then says: "Now that I know that, I know what the numbers are."
And finally the sum guy says: "Now that I know that, I know what the numbers are too."

What are the numbers?

EDIT: OMG You guys are gonna hate me for this but I wrote the problem wrong! I mixed up the sum and product guys! So sorry it took this long for me to notice! I Already fixed it and will post it again!

Yeah, Trucko you're correct, even though I forgot to mention 210 in the 10 digit system. Well done.

As for your Johann, that seems darn hard. Is the second last sentence a typo and should be product, or does he talk twice?

That actualy was a typo. Thanks for pointin it out. It`s already fixed.