Question 1033859
The model to use is {{{m = m[0]*0.98^t}}}where m is the mass remaining after t years, and {{{m[0] }}} is the original mass of the substance.

To find the half-life, set {{{m = m[0]/2}}}.

==> {{{m[0]/2 = m[0]*0.98^t}}} ==> {{{1/2 = 0.98^t}}}

==> {{{t = -ln2/ln0.98 = 34.3}}}years.