A dart board is in the shape of an equilateral triangle of side 20cm. 5 darts are thrown at random on the board, what is the probability that there are at least 2 darts whose distance apart is at most 10cm ?

Depends on the thrower.
1 thrower may not hit the board, another may split the darts. It is a skill, not a chance game.

What do you think is the answer? Use this:

Probability at least 2 darts are 10 cm apart = 1 - Probability no 2 darts are within and including 10 cm apart.

Is there a way to get all 5 darts at more than 10 cm apart? That is the real question.