Radon has a half-life of 56s. After what time will it's activity be reduced to (a) 90% (b) 50% (c) 10% of it's initial value​

Aplane flying horizontally at an altitude of 1 mile and a speed of of 500mih passes directly over a radar station. find the rate at which the distance from the plane to the station is increasing when it is 2mi away from the station.