8) if during the last 100 feet of approaching a railroad crossing, you cannot see the track for at least 400 feet in both directions, you may not cross the tracks at a speed faster than mph.

15 MPH

Explanation:

If one can't see the track for at least 400 feet in both directions within the last 100 feet of approaching a railroad crossing, one won't be able to cross the tracks at a speed faster than 15mph

It's so because that is the speed limit within 100 feet of a railroad crossing where one can't see the tracks for 400 feet in both directions.

Although, one can travel more than that speed if the crossing is being controlled by gates or flagman.

the answer is b ) square root of 5