Question

Dudley is going on a road trip from Washington, DC to St. Louis, MO and back in his Batmobile. Luckily, this is exactly a 500 mile drive. He completed the Washington, DC to St. Louis leg of his trip going 50 mph. His speedometer died on the way back, but miraculously, Dudley is a genius and knows that his average speed for the entire trip was 90mph. How fast was Dudley going (in miles per hour) on the way back, assuming he traveled at a constant speed?

Answer 1

1. find total time.
2. find time taken on the way back.
3. calculate speed on the way back.

can you try something along these lines ?

Answer 2

d_1 = 50(t_1)
250 = 50t_1
t_1 = 5

Answer 3

t_1 = 5 is time taken from Washington to St. Louis
Can you make use of that 90 mph and find out total time ?

Answer 4

Um, I'm not sure what to do there. I got stuck around here last time I tried.

Answer 5

average speed = total distance / total time
we know av speed = 90 mph
we know total distance = 500 miles
So, total time = ?

Now use total time and t_1 to calculate time taken in returning

Answer 6

90 = 500/x
x = 500/90
x = 50/9

Answer 7

50/9 - 5 = 5/9

Answer 8

yeah thats the time taken for the return journey.

know you know time for returning, you know the distance from St Louis to Washington. CAn you calculate speed when returning ?

Answer 9

s = d/t
s = 250/(5/9)
s = 450

Answer 10

Alright, thank you! THat's the given answer. I think I just forgot that average speed = total distance/total time.