A man goes from C to D at 40 km/h and he returns from D to C at x km/h. If the average speed of the man for the whole journey is 60 km/h, then what is the value of x?

120

Average Speed = \(\frac{ 2 × S1 × S2}{S1 + S2}\)

S1 = 40 km/h & Average speed = 60 km/h  is given

60 = \(\frac{ 2 × 40 × S2}{40 + S2}\)

60 × ( 4 + S2 ) = 80 S2

240 + 60 ×S2 = 80× S2

20×S2 = 240 

S2 = 120 km/h

So , Speed while returning back is 120 km/h