A driver has a journey of 300km to complete. On one particular journey she travelled 10/h faster than her usual average speed and her journey took one hour less. What is her usual average speed?

Answer 1

If it normally takes the driver T hours to travel 300 km, then the average speed is 300/T km/hr.

Since it takes the driver T - 1 hours when traveling (300/T) + 10 km/hr to cover the same distance:

300 = (300+10T)/T * (T - 1) = (300T + 10T² - 10T - 300)/T

300T = 300T + 10T² -10T - 300

0 = 10T² - 10T - 300 = T² - T - 30 = (T+5)(T-6)

Since T cannot be negative, T=6. Thus her normal average speed is 300/6 or 50 kph.