In 10 hours 75% mass of a radio active sample is converted into a stable element. Then mean life of the radio active element is

If m0 be the original mass of radio active element and T be its half life, then after 10 hours mass of remaining radio active element will be m0/4∴ m04=m0210T⇒122=1210/T⇒10T=2⇒T=5 hrsIf half life is T, then T1=Tln2 =50.693hr=7.215 hr