Question 1031278
The working equation is {{{m = 90e^(rt)}}}

==> {{{45 = 90e^(5r)}}} ==> {{{r = -ln2/5}}}

==>  {{{m = 90e^((-ln2/5)t) = 90(e^(ln2))^(-t/5) = 90*2^(-t/5) = 90*(1/2)^(t/5) }}}