Find the half life

Question

the half life of a radioactive substance is 105 years. How long will it take 75 grams to decay to 13 grams?

a 265.5 years

b 304.8 years

c 221.4 years

d 198.7 years

Answer 1

13 = 75(0.5)^t/105

0.1733 = 0.5^t/105

ln 0.1733 = (t/105) ln 0.5

Can you solve for t and answer?