Question

Annabelle's taking the Polar Express all the way to the North Pole The Polar Express is 100 m long and it takes 30 seconds to cross and his 400m rice Bridge what speed is the train going at

Answer 1

Total distance is calculated by taking summation of Polar express and the rice bridge. So Total distance is 500 m which is 0.5 Km. Time is 30 sec which when converts to hours= 30/3600= 0.0083 hrs. So Speed= Distnace/ Time = 0.5/0.0083 =60.2km/hr Hence in this case the speed of train is calculated to be 60.2 Km/hr.

Answer 2

Given that the length of the train is 100 m when the train covers the 400 m of the bridge; the length o the train, 100 m, is still on the bridge. The actual distance that the train covers to cross the bridge is 400 m (the length of the bridge) plus 100 m (the length of the train), which is equal to 500 m. 
Now, $500m( \frac{1km}{1000m} )= \frac{1}{2} km$, and $30s( \frac{1h}{3600s} )= \frac{1}{120} h$

We know that $d=vt$, so $v= \frac{d}{t}$.And for our previous calculation we also know that $d= \frac{1}{2} km$ and $t= \frac{1}{120} km$. The only thing left is replacing those values in our equation:

$v= \frac{ \frac{1}{2}km }{ \frac{1}{120} h}$
$v= \frac{120km}{2h}$
$v=60kmh$

We can conclude that the speed of the train is 60 kmh.