A man completes his journey in 6 hours. He covers half the distance at 10 km/h and the rest at 20 km/h. The length of the journey is:

80 km

Average speed of the whole journey =  \(\frac{ 2 × S1 ×S2 }{S1 + S2 }\) 

= \(\frac{ 2 × 20 ×10 }{20 + 10 }\)

= \(\frac{ 400 }{30 }\) = \(\frac{ 80 }{6 }\)

Total Distance  = \(\frac{ 80 }{6 }\) × 6 = 80 km