Question

Johnny was able to drive an average of 29 miles per hour faster on his car after the traffic cleared. He drove 30 miles in traffic before it cleared and the drove another 176 miles. If the total trip took 6 hours, then what was his average speed in traffic?

Answer 1

15 mph

Explanation:

Before traffic cleared:

distance = 30 miles

average speed = s

time = t

d = st

30 = st

After traffic cleared:

distance = 176 miles

average speed = s + 29

time = 6 - t

d = st

176 = (s + 29)(6 - t)

30 = st

s = 30/t

176 = (30/t + 29)(6 - t)

176t = (30 + 29t)(6 - t)

176t = 180 - 30t + 174t - 29t^2

29t^2 + 32t - 180 = 0

(29t + 90)(t - 2) = 0

t = -90/29 or t = 2

Time must be a positive number, so we discard the solution t = -90/29.

t = 2

30 = st

30 = s * 2

s = 15

Answer: 15 mph