Question

Car m was driving on a highway at a constant speed of 50 miles per hour. If car n was 3 miles behind car m 20 minutes ago and 2 miles ahead of car m 5 minutes ago, what was the speed of car n? Assume that car n was driving at a constant speed.

Answer 1

Let the speed of car n is v miles /hr

a/c to question,

speed of car m is 50 miles/hr.

car n was 3 miles behind car m 20 minutes ago and 2 miles ahead of car m 5 minutes ago.

during 15 minutes , relative distance of car n becomes (3 + 2) = 5miles.

so, relative distance = relative velocity × time taken

or, 5 miles = velocity of car n with respect to car m × 15 min

or, 5miles = (v - 50miles/hr) × 15/60 hr

or, 5 = (v - 50) × 1/4

or, 20 = v - 50

or, v = 20 + 50 = 70 miles/hr

hence, speed of car n is 70 miles/hr