Question 1026192
{{{I = I[0]e^(-0.51t)}}}

When {{{I[0] = 25}}} and I = 2.5, we have {{{2.5 = 25e^(-0.51t)}}}. 

==> {{{e^(0.51t) = 25/2.5 = 10}}}

==> 0.51t = ln10 ==> {{{t = ln10/0.51 = 4.5}}}seconds.