A group of muons with a half life of 1.5 mu sec are produce at a height of 30 kilometre above the surface of the earth how long would it take the group travelli

a group of muons with a half life of 1.5 mu sec are produce at a height of 30 kilometre above the surface of the earth how long would it take the group travelling vertically downward to reach the earth and what fraction of them would remain undecayed by them