A man travels 75 km at speed 25 km/h and next 60 km at 20 km/h and there after travel 90 km at 15 km/h. His average speed is:

18.75 km/h

Average Speed =  \(\frac{Total \;DIstance }{Total \;TIme }\)

Time 1 =  \(\frac{DIstance}{Speed}\)

=  \(\frac{75}{25}\)

= 3 hours

Time 2 =  \(\frac{DIstance}{Speed}\)

=  \(\frac{60}{20}\)

= 3 hours

Time 3 =  \(\frac{DIstance}{Speed}\)

=  \(\frac{90}{15}\)

= 6 hours

Total time = 3 + 3 + 6 = 12 hours

Total distance = 75 + 60 + 90 = 225 km

Average speed =  \(\frac{225}{12}\)

= 18.75 km/h