Question 458292
After some cumbersome algebra, the formula for the exponential growth is given by 

{{{B(t) = 400*(15/4)^((t-10)/25)}}}.

The initial number is given by B(0) = {{{400/(15/4)^0.4}}}

To find the doubling period, {{{800/(15/4)^0.4 = (400/(15/4)^0.4)*(15/4)^((t-10)/25)}}}

==> {{{2 = (15/4)^((t-10)/25)}}}
==> {{{ln2 = ((t-10)/25)*ln(15/4)}}}

==> {{{(25 ln2)/ln(15/4) = t- 10}}}

==> {{{(25 ln2)/ln(15/4) + 10= t}}}

==> t = 23.11 minutes, approximately.