1.  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 ?
A. 19
B. 28
C. 30
D. 37

Answer: Option C

Explanation:

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 x 57 = 19.
3

So, their paths cross at (11 +19) i.e., 30th floor.