In a two digit positive number, the digit in the units place is equal to the square of the digit in tens place, and the difference between the number and the number obtained by interchanging the digits is 54. What is 40 % of the original number ?
A train running at a speed of 40 m/s crosses a pole in 21 sec less than the time it requires to cross a bridge 3.5 times its length at the same speed. What is the length of the bridge ?
a) 240 m
b) 420 m
c) 560 m
d) 840 m