Krypton-81 half life is 13s. How long will it take the activity of this isotope to reach 1/4 original value?

Answer 1

ok so you HAVE to assume the original value is 1 so this is how you do it ok.

0.25=1(0.5)^t/13
then you do
log0.25/log(1 x 0.5)=t/13
then whatever answer you get (i dont have a calc. near me) so lets call the answer xyz so the next step is:
(xyz) X 13= The time required to reach the 1/4 of its original value.

Answer 2

After 13 sec, it will be half its original value. After 26 sec (2 x 13 sec) it will be 1/4 its original value. It gets halved every 13 sec.