A radioactive element decays someone please help show me how to work this out

Question

A radioactive element decays into non-radioactive substances. In 980980 days the radioactivity of a sample decreases by 6565 percent. Give your answer to the following questions round to the nearest whole number of days.

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

half-life:    (days)

(b) How long will it take for a sample of 100100 mg to decay to 9595 mg?

time needed:    (days)

Answer 1

a) 647 b) 48

a) t= 980ln.5/ln.35 = about 647 days = half time when it decays 50% from 1000 to 500 mg

b) t= 980ln.95/ln.35 = about 48 days = time when it decays 5% from 1000 to 950 mg

if it decays 65% in 980 days, leaving 35%