A traveller covers 90 km in 3 hours, partly by car at 40 km/h and partly by bicycle at 15 km/h. How far does he go by car?

Explanation

1. 01

   The mean speed is $\frac{90}{3} = 30$ km/h. Alligation on speeds gives the ratio of times: bicycle : car $= (40 - 30) : (30 - 15) = 2 : 3$.

2. 02

   Time by car $= \frac{3}{5} \times 3 = 1.8$ hours, so the distance by car is $1.8 \times 40 = 72$ km.