half-life word problem

Question

An unknown radioactive element decays into non-radioactive substances. In 980 days the radioactivity of a sample decreases by 52 percent.

(a) What is the half-life of the element?
half-life:

(b) How long will it take for a sample of 100 mg to decay to 73 mg?
time needed:

Answer 1

A = a₀(0.5)^t/h

0.48a₀ = a₀(0.5)^980/h           52% decrease means 48% remains

0.48 = (0.5)^980/h

ln 0.48 = (980/h) ln 0.5

-0.733969 ≈ (980)/h) (-0.693147)

1.05889 ≈ 980 / h

h ≈ 925.49

 

Use the same formula/process to answer (b).