Stuck on this kinetics problem...

Question

Sr is one of the harmful radionuclides resulting from nuclear fission explosions. It decays by a 1st order process with a half life of 28 years. How many yrs would it take for 99.99% of a given sample released in the atmosphere to disintegrate? in units of years

Answer 1

log [A2] - log [A1] = - 0.3009 t2 / half life

log [0.01] - log [100] = - 0.3009 t2 / 28

-2 -(2) = - 0.3009 t2 / 28

t = 4 (28) /0.3009

t = 372.2 years

I will leave the rounding off to you.  you had four sig figs in the "99.99", but only two in the "28"  could the 28 had any decimals