The half-life of rhodium, rh-106, is about 30 seconds. you start with 500 grams.

I think the logical question for this would be to find the reaction rate constant, k, for radioactive decay of Rhodium. The general formula is:

ln A/A₀ = -kt,
where 
A is the amount of substance after time t,
A₀ is the original amount of substance
t is the time

After 30 s, the amount of Rhodium would be in half. So, A/A₀ = 1/2

ln(1/2) = -k(30 s)
k = 0.023 s⁻¹