A sample of radioactive element contains 5×1010 active nuclei. If half life of element is 10 days, then the number of decayed nuclei after 40 days is

Number of half lives =n=tT12=4010=4  Decay Law, NN0=12n ⇒N=N012n ⇒N=5×1010×124=5×101016=0.3125×1010 Thus,  number of nuclei decayed after 40 days=N0−N= 5×1010−0.3125×1010=4.6875×1010