David gets on the elevator at the 11th floor of a building and rides up at the rate of 57 floors per minute. At the same time, Albert gets on an elevator at the 51st floor of the same building and rides down at the rate of 63 floors per minute. If they continue travelling at these rates, then at which floor will their paths cross ?	Choose the most appropriate option
A) 15	B) 20	C) 25	D) 35	E) 30
Suppose their paths cross after x minutes.   Then, 11 + 57x = 51 - 63x = 120x = 40   x = 1/3   Number of floors covered by David in (1/3) min. = (1/3 x 57)= 19.   So, their paths cross at (11 +19) i.e., 30th floor.   Answer is 30.
<result>E</result>
David gets on the elevator at the 11th floor of a building and rides up at the rate of 57 floors per minute. At the same time, Albert gets on an elevator at the 51st floor of the same building and rides down at the rate of 63 floors per minute. If they continue travelling at these rates, then at which floor will their paths cross ?