Question

The tail of a 1-mile long train exits a tunnel exactly 3 minutes after the front of the train entered the tunnel. If the train is moving 60 miles per hour, how many miles long is the tunnel?

Answer 1

60 mph is One mile a minute. So the train will start to exit the tunnel after 7 minutes. It will finish exiting the tunnel 1.5 mins later.

Answer 2

The tail of the train starts one mile from the mouth of the tunnel and it must travel that distance plus the length of the tunnel in 3 minutes.

 

Let t = tunnel length

 

t + 1  = distance the tail must travel in 3 minutes.

3 minutes = 1/20 hr

 

 (t+1) miles  /  60 m/hr  = 1/20 hr

t + 1 = 60/20

t+1 = 3

Subtract 1 from each side:

t = 2

The tunnel is 2 miles long.