A train traveling at 100 km an hour takes 3 seconds to enter a tunnel and an additional thirty-five seconds to pass completely through it. Find the length, in kilometers, of the train. Express your answer as a common fraction.
The train takes 3 seconds at 100km to enter the tunnel
3 sec / 3600 sec/hr x 100 km/ hr = 1/12 km long train
The tail (caboose) of the train takes 3 seconds + 35 seconds to pass through the tunnel
38 seconds / 3600 sec/hr x 100 km/hr = 1.0555 km
the train length is 3 out of 38 of this = 3/38 x 1.0555=.08333 km = 1/12 km long