11-12 Two trains; one going to east, and the other is heading north

$s = \sqrt{1600t^2 + 2500(t - 2)^2}$

$\dfrac{ds}{dt} = \dfrac{3200t + 5000(t - 2)}{2\sqrt{1600t^2 + 2500(t - 2)^2}}$

$\dfrac{ds}{dt} = \dfrac{4100t - 5000}{\sqrt{1600t^2 + 2500(t - 2)^2}}$
 

at 3 PM, t = 3
$\dfrac{ds}{dt} = \dfrac{4100(3) - 5000}{\sqrt{1600(3^2) + 2500(3 - 2)^2}}$

$\dfrac{ds}{dt} = 56.15 \, \text{ mi/hr}$           answer

$\dfrac{ds}{dt} = \dfrac{4100t - 5000}{\sqrt{1600t^2 + 2500t^2 - 10\,000t + 10\,000}}$

$\dfrac{ds}{dt} = \dfrac{4100t - 5000}{\sqrt{4100t^2 - 10\,000t + 10\,000}} \times \dfrac{1/t}{1/t}$

$\dfrac{ds}{dt} = \dfrac{4100 - 5000/t}{\sqrt{4100 - 10\,000/t + 10\,000/t^2}}$

$\dfrac{ds}{dt} = \dfrac{4100 - 5000/\infty}{\sqrt{4100 - 10\,000/\infty + 10\,000/\infty^2}}$

$\dfrac{ds}{dt} = 64.03 \, \text{ mi/hr}$           answer