Aptitude_Q&A_02 
21.  In a flight of 600 km, an aircraft was slowed down due
to bad weather. Its average speed for the trip was reduced
by 200 km/hr and the time of flight increased by 30 minutes.
What is the duration of the flight?
 
Solution: 3 hrs. and 4 hrs. are too large as the average speed is to be greater than 200 kmph. Test 1 hr. Time is 1 hr., so Normal speed is 600 kmph. Slower speed is 400 kmph. and time is 1 hr 30 min. So flight time increase is 30 min. and this fits the bill. 
22.  A man completes a journey in 10 hours. He travels first half of the journey at the rate of 21 km/hr and second half at the rate of 24 km/hr. Find the total journey in km.  
Solution: [Note the simplification / solution below!]  
Let half the distance be d km. d is calculated mentally! [Explanation below.] Equating total travel time: Multiply everything by 3: Multiply everything by 7 x 8: 8d + 7d = 1680 Solving d = 112 Total journey is 224 km.  
An Alternative Solution:  
'An Useful RULE': If two equal distances are covered at two different speeds s_{1} and s_{2} the average speed for the whole (double distance) journey is  
Solution: Average speed for the total, 10 hrs. journey, is 22.4 kmph; Total journey is 22.4 x 10 = 224 km. 
23.  The ratio between the speeds of two trains is
7 : 8. If the second train runs 400 km in 4 hours, then the speed
of the first train is: 70 kmph., 2) 75 kmph., 3) 84 kmph., 4) 87.5 kmph. 
Solution: Speed of 2nd train is 400/4 = 100 kmph. Using the ratio, speed of 1st train 7 of 8 parts; seveneighth of 100 = 87.5 kmph.; 
24.  A man on tour travels first 160 km at 64 km/hr and the next 160 km at 80 km/hr. What is the average speed for the first 320 km of the tour?  
Solution:  
Equal distances at different speeds:  

25. 
 
Solution: OneStep Solution! The beauty of the Calculation!! ; 