A Nuclear reactor produces product 20kg of uranium-235. if the half life is about 70years, how long will it take to decay to 0.1kg

t = (70) ln (19.9/20) / -ln2

70 is the years. 19.9 kg is what is left after 0.1kg decay. 20kg is your initial amount.

t=0.506 years with a decay constant of 0.0099021/year