Question

a boy runs for 10 min at a uniform speed of 9 km/h. At what speed should he run for the next 20 min so that the average speed comes 12 km/h

Answer 1

speed₁ = 9 km/h = 150 m/min 
time₁ = 10 min 
speed₂ = x m/min
time₂ = 20 min 
average speed = 12 km/h = 200 m/min
therefore ,
average = total speed / total time 
200 = 150 + x / 20 + 10
200 = 150 +x / 30 
200 * 30 = 150 + x
6000 = 150 +x 
6000 - 150 = x
5850 = x 
therefore the speed is 5850 m/min = 351 km/h
the answer is 351 km/h
   i hope it helps.......................^_^