20% of a journey is covered at a speed of 80 km/h, 25% of the journey at a speed of 100 km/h and the rest at 110 km/h.  What is the average speed (in km/h) for the whole journey?

100

Let total distance = 100 km

Total time = \(\frac{20}{80}\) + \(\frac{25}{100}\) + \(\frac{55}{110}\)

                = \(\frac{1}{4}\) + \(\frac{1}{4}\) + \(\frac{1}{2}\)

                = 1

         Avg. speed = \(\frac{Total\;distance}{Total\;time}\)

                          = \(\frac{100}{1}\)

                           = 100 km/h