When the airplane moves at an average speed of 450 kmph it reaches its destination on time. When its average speed becomes 400 kmph, then it reaches its destination 30 minutes late. Find the length of journey.	Choose the correct choice from the following
A) 1250 km
B) 1500 km
C) 1100 km
D) 1800 km
E) None
Sol.   Difference between timings = 30 min = 1/2 hr.   Let the length of journey be x km.   Then, x/400 - x/450 = 1/2   â‡” 9x - 8x = 1800   â‡” x = 1800 km.   Answer 1800 km
<result>D</result>
When the airplane moves at an average speed of 450 kmph it reaches its destination on time. When its average speed becomes 400 kmph, then it reaches its destination 30 minutes late. Find the length of journey.