Chemistry
posted by John .
A radioactive element has a halflife of 6.41 days. What percent of the original sample will be left after 15.0 days?

estimate: that is over 2 half lives, guessing 20 percent. The math
amountremaining=original*e^(ln2*15/6.42)
decimal percent=e^(ln2*15/6.42)=.199
or 19.9 percent 
Thanks