Carbon (Z=6) with mass number 11 decays to boron (Z=5).(a) Is it a `beta^+` -decay? (b) the half-life of the decay scheme is 20.3 minutes .How much time will elapse before a mixture of 90% carbon-11 and 10% boron-11(by the number pf atoms )converts itself into a mixture of 10% carbon-11 and 90% boron -11?