Quiz-1

Speed = 600/ 5 x 60 m/sec. = 2 m/sec. = 2 x 18/5km/hr =7.2 km/hr

Distance = (240 x 5) = 1200 km.
Speed = Distance/Time
Speed = 1200/(5/3) km/hr.
Required speed = 1200 x 3/5km/hr = 720 km/hr.

Initial speed = 80km/hr
Total distance = 80 * 10 = 800km
new speed = 800/4 =200km/hr
Increase in speed = 200 - 80 = 120km/hr

Each of the two periods of 8 hours and 11 hours consists of two component time periods, one upstream at a speed that is the difference in boat speed and stream speed, and the other downstream at a speed that is a sum of boat speed and stream speed

Relative speed of the thief and policeman = (11 – 10) km/hr = 1 km/hr
Distance covered in 6 minutes = 160*6=1/10=100meters
So distance between them after 6 minutes = 200 - 100= 100 meters

Let the speeds of two trains be 7X and 8X km/hr.
8X=400/4=>X=12.5Km/hr
So speed of first train is 12.5*7 = 87.5 km/hr

You can write, X = 81 km/h
Therefore, speed in m/s (according to our formula) = X x 5/18
= (81 x 5/18)
= 22.5 m/s

Due to stoppages, it covers 9 km less.
Time taken to cover 9 km = 9/54 x 60 min = 10 min.

160 miles= 330 min
1 mile = 330/160
80 miles=(330*80)/160=165 min.

Since the distance travelled on both sides is the same, we can use the formula of harmonic mean of speeds
Average Speed: 2xy/(x+y) where, x is the speed while going from A to B and y is the speed while coming back.
So using this formula, we get the answer as 48 kmph.