Out of how many people should a group at least consist in order to have that the probability that two persons out of that group are having birthday on the same day, is larger than 1/2?

A wire with a length of 60 meter is hanging between two high voltage pylons. The wire is connected to the pylons at a height of 50 meters. The middle part of the wire is 20 meters above ground level. How far are the high voltage pylons standing apart?

How should you saw a block of stone of 40 kg into four pieces such that you can measure all weights of an integer from 1 kg to 40 kg (so 1 kg, 2 kg, 3 kg, 4 kg, ..... 39 kg and 40 kg) on a balance?