A car covers 15 km, 20 km, 30 km and 12 km at speeds of 20 km/h, 30 km/h, 40 km/h and 30 km/h, respectively. The average speed of the car for the total journey is :

30 km/h

Time = \(\frac{Distance }{Speed}\)

Total time taken = \(\frac{15 }{20}\) + \(\frac{20 }{30}\) + \(\frac{30 }{40}\) + \(\frac{12 }{30}\)

= \(\frac{3 }{4}\) + \(\frac{2 }{3}\) +  \(\frac{3}{4}\) + \(\frac{2 }{5}\)

= \(\frac{45 + 40+ 45 + 24 }{60}\)

= \(\frac{ 154}{60}\)

Average Speed = \(\frac{Total Distance  }{Total Time }\)

= \(\frac{ 77 × 60  }{154 }\)

= 30 km/h