Half life is the period of time after which half amount of element is decayed.
Half life of rhodium = 30 sec
Amount of rhodium = 500 grams
how to write the equation, if we know the half life and starting or initial amount of rhodium is:
N = 500 g (1/2)^(t / 30 sec)
t is the time and
N is the amount of rhodium left at time t.