Half life??? thanks :)

Question

After 40.5 days, a 3.84g sample of I-131 decays down to 0.120g. What is its half-life?

Answer 1

Rate of decay :

3.84 - 0.12 =3.72

3.72 / 40.5 = 0.0919 grams per day (roughly)

Half life is point at which half remains, so 3.84/2 = 1.92 grams

1.92 / 0.0919 = 20.8 days

Answer 2

let.s observe: 3.84g/0.12g=32 in the end the sample is 1/32 times than initialy.this means it  divided in half  5 times.so half life is 40.5 day/5=8.1day.

DW is stupid.this is a word of caution.