2nd puzzle by Dr Harry Wiggins (Helping Hercules)
Hydra is a serpentine water monster that lives in the lake of Lerna. It is Hercules's task to kill this monster (the second task of his Twelve Labors). The Hydra possesses 100 heads and can only be killed by cutting of ALL his heads. It is know that you can cut off exactly 10, 20, 30 or 40 heads at a time. However with each of these 40, 2, 50 or 4 new heads will re-appear in each case.
How can Hercules defeat the monster? What is the least number of cuts Hercules needs?