Two trains, with traveling speed 50 km/h (mph), are going against each other, on the same rail, so they are going to crash. When the distance between them is 100km (miles), a superfly starts to fly with the amazing speed of 100 km/h (mph) towards the other train. When the superfly reaches the other train, she makes a 180 degrees turn and flies back to the first train, then she makes a turn and flies towards the other train, then ….. and so on, until she is crushed by the colliding trains. The question is how many kilometers (miles) flies the superfly, before she dies? (superfly turns really fast, so she always fly 100km/h (mph))

You can get the answer, by doing some advanced math, by calculating the distaces at the times, when superfly reaches a train and use infinite number sequences to count the distance, or some simply math like:

the distance between them is 100km (miles),
trains are equaly fast, so they collide in the middle
after traveling 50km (miles). Their speed si 50km/h (mph),
so they will collide in one hour. The speed of the superfly
is 100 km/h (mph), so she trawels 100 km (miles), before she is dead.