A delivery vehicle covers D/4 with speed v, next D/4 with speed 2v and remaining D/2 with speed 3v. During the journey it remains stationary for time D/(12v). Its average speed is:
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A delivery vehicle covers D/4 with speed v, next D/4 with speed 2v and remaining D/2 with speed 3v. During the journey it remains stationary for time D/(12v). Its average speed is:

G1 Physics · Accepted Answer

Moving time = D/(4v) + D/(8v) + D/(6v). Including the rest time D/(12v), Total time = D/v [1/4 + 1/8 + 1/6 + 1/12] = D/v [6/24 + 3/24 + 4/24 + 2/24] = 5D/(8v). Thus average speed = D/[5D/(8v)] = 8v/5.