Question

1] a car covers the first half of the distance between two places at a speed of 40 km/h and the second half at 60 km/h .what is the average speed of the car

2]a car goes from town A to another town B with a speed of 40 km/h and returns back to the town A with a speed of 60 km/h.calculate the average speed during the whole journey.

3] a girl moves with a speed of 6 km/h for 2hrs and with the speed of 4km/h for the next 3hrs.
  Find the average speed of the girl and the total distance moved.

Answer 1

(1) let x/2 distance cover in time t1 & another x/2 distance cover in time t2
t1 = (x/2)/40
t2 = (x/2)/60
t1+t2 = x/48
average speed = x/(t1+t2) 
                     = x/(x/48)
                     = 48Km/hr
(2) as the above
t1 = x/40
t2 = x/60
t1+t2 = x/24
average speed = 2x/(x/24)
                      =48Km/hr 
(3) s1 = 6*2 
           = 12Km.
s2 = 4*3
      = 12Km.
total distance = s1+s2 = 24km.
average speed = 24/5
                      = 4.8Km/hr

Answer 2

the car pass the same distance... so we can use the formal
 2xy/x+y
in this x is avg speed 1.. and y is avg speed 2..
so 2*40*60/40+60
=4800/100
=48km/hr is answer
2nd one is also the same formate... distance are same ... so we aply the same formal
and get the answers
is  2*40*60/40+60
=4800/100
=48km/hr is answer


3. the girl travel 6km/hr for 2hours
4km/hr for 3hr
so the distanece is
6*2=12
4*3=12
so travel the same distance 
so averge speed by calculating using the formala
2xy/x+y
2*6*4/6+4
=48/10
=4.8km/hr
distance travel is 12+12=24km