Question

Sam drove for 2 hours at 45 mph. He has the same distance remaining. How fast should he drive to make his average speed 60 mph?

Answer 1

Speed (S) = Distance (D) / time (t)

First stretch

45 = D / 2

D = 45*2

D = 90 mp

How is the same distance:

Second stretch

S = 90 / t

Average speed (AS):

AS = (S1+S2) / 2

60 = [45+(90/t)] / 2

120 = 45+90/t

120-45 = 90/t

75 = 90/t

t = 90/75

t = 1,2 h

________________________

And now:

S = 90 / t

S = 90 / 1,2

S = 75 mph