At 2:30 pm, Bob heads north from an intersection at 120 km/h. Alvin is driving east at 96 km/h and reaches the same destination at 5:10 pm. When are they closest to each other.

$\displaystyle d(t)=\sqrt{(120t)^2+(256-96t)^2}$

Is that correct? The answer is supposedly 3:32 PM, so I'm not sure if I understood it incorrectly or if the book had a typo.

Thanks for the assistance!