The master needs some of the trees (twenty, to be exact) at the back of his spooky old mansion cleared to make way for a new evil laboratory, so he decides to send some his slaves to do the work for him.
He initially sends out four of his men, armed with axes, to chop the trees down. Due to the fact he is very impatient, every ten minutes he sends out another man to help with the work.
Assuming that it takes one man 30 minutes to chop down 1/3 of a tree, how long till all twenty trees are chopped down?
