just did a question just like yours
http://www.jiskha.com/display.cgi?id=1448296657
follow the steps
for the half life, set the amount = 50 and solve for t
The amount remaining of a radioactive substance after t hours is given by
A(t) = 100ekt. After 12 hours, the initial amount has decreased by 4%.
How much remains after 48 hours?
What is the half-life of the substance?
2 answers
The amout remaining of a radioactive substance after t hours id given by A(t)=100e^kt. After12hours, the initial amount had decreased by 7%