The halflife of Phosphorus is 14.3 days, if i start with 100g how long will it take so that only 25g will be left?

One month is my answer, I just need confirmation. Thank you

2 answers

amount left=original*(1/2)^(days/14.3)
25/100=(1/2)^(days/14.3)
(1/2)^2=(1/2)^(days/14.3)
so days=28.6
Thank you