Answer
(a) $0.7565$ hours
(b) $156233$
Work Step by Step
(a) Given $n_0=10000,n(1)=25000$, use the doubling time model $n(t)=n_0\cdot 2^{t/a}$ to get
$n(1)=10000\times2^{1/a}=25000$ which gives $a=\frac{1}{log_2(25000/10000)}=0.7565$ hours
(b) With $t=3$, we have $n(3)=10000\times2^{3/0.7565}\approx156233$
