A bus covers 3 equal distances. The first is covered with speed 10 km/h, second with speed 20 km/h and third with 60 km/h, then find its average speed in km/h.

Options

A. 9
B. 16
C. 18
D. 60

Answer

C

Explanation

Suppose total distacne is 3d and distance in each part is d.
Time taken to cover `1 ^(sth)` part, `t _(1) = (d)/(10)`
Time taken to cover `2^(nd)` pard, `t _(2) = (d)/(20)`
Time taken to cover `3^(rd)` part, `t _(3) =(d )/(30)`
Total time taken, `t =(3d)/(lt v gt )`
`therefore t = t _(1) + t _(2) + t _(3)`
`(3d)/(lt v gt) = (d)/(10) + (d)/(20) + (d)/(60)`
`therefore(3)/(lt v gt) = (6 + 3 +1)/(60)`
`therefore (3)/(lt v gt) = (10)/(60)`
`therefore lt v gt = (3 xx 60)/(10)`
`therefore lt v gt = (180)/(10)`
`therefore lt v gt = 18 km//h`