The formula for decay is
N' = Ne^(-kt)
with N as the initial amount, k as the decay constant, and N' as the final amount. For decay, 280 days is the time it takes for the initial amount N to decay to 1/2N (half its original mass).
1/2A = Ae^(-280k)
1/2 = e^(-280k)
Solving for k, we will yield
ln(1/2) = -280k
k = ln(2) / 280 day^-1
k ≈ 2.5 × 10^-3 day^-1
