Tritium decays by a first order process that has half-life of 12.5years. how many years will it take to reduce the radio activity of a tritium sample to 15% of it's original value.

Answer 1

k = 0.693/half life = ?
Then ln(No/N) = kt
ln(100/85) = kt
plug k in and solve for time in years.
Post your work if you get stuck.