Question

if motorist drove a distance of 90km at constant speed for the first 40km and then drove 30 km/hr faster for the rest of the journey if the journey took him one hour, his average speed for the first 40 km is​

Answer 1

Explanation:

The motorist drove at a distance of 90km at a constant speed for the first 40km, and then 20 km/hr faster for the rest of the journey.

The whole journey took him 1hr.

Let S denotes the average speed (in km/hr) for the first 40 km.

Hence we get the following relation,

40/S + (90 - 40)/(S + 20) = 1

or 50/(S + 20) = 1 - 40/S = (S - 40)/S

or (S + 20)(S - 40) = 50S or S^2 - 70S - 800 = 0 or (S - 80)(S + 10) = 0

or S = 80 (km/hr) [Ans]

[S cannot be negative value]