# Finding time from average speed

## Homework Statement

A golfer rides in a golf cart at an average speed of 3.1 m/s for 28s. She then gets out of the cart and starts walking at an average speed of 1.3 m/s. For how long must she walk if her average speed for the entire trip, riding and walking, is 1.8 m/s ?

## Homework Equations

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Vave = distance/time

## The Attempt at a Solution

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This question seems simple considering there's only formula at play. I tried splitting up into 2 parts. The first part, the person walks at an average speed of 3.1 m/s for 28s. Distance can be calculated as 86.8 m

After this I got lost. I know there's a second part where she walks at an average speed of 1.3 m/s. However nothing else is known. And we know her entire average speed is 1.8 m/s

Any ideas?

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Samy_A
Homework Helper
Let ##t## be the time she walks.
You can compute the total time of the trip and the total distance of the trip in function of ##t##.
As you also know the average speed for the entire trip, this will give you an equation in ##t##.

I didn't quite understand your second line.
Right now I have 1.8=86.8 + d / 28 + t where 1.8 is the average speed of the whole trip, d is the unknown distance from the second interval, and t is the unknown time from the second interval.

Im not sure if im on the right track nor where to proceed from here

EDIT
where does the 1.3 m/s speed in the second interval come into play?

Samy_A
Homework Helper
I didn't quite understand your second line.
Right now I have 1.8=86.8 + d / 28 + t where 1.8 is the average speed of the whole trip, d is the unknown distance from the second interval, and t is the unknown time from the second interval.

Im not sure if im on the right track nor where to proceed from here
I assume you meant ##1.8=(86.8 + d) /( 28 + t)##.

You know that she walked at 1.3 m/s, so that should allow you to express ##d## in terms of ##t##.

jbriggs444
Homework Helper
2019 Award
Minor correction: she walks at 1.3 m/s

Samy_A
Homework Helper
Minor correction: she walks at 1.3 m/s
Thanks, I corrected the typo.

Samy_A
Homework Helper
EDIT
where does the 1.3 m/s speed in the second interval come into play?
That's the correct question. You haven't used the 1.3 m/s speed so far.
What relation is there between 1.3 m/s, d and t?

Ah I think i got it now.

Right now we have the following: 1.8=(86.8+d)/(28+t).

If we find that d then we can have an equation solvable for t.

In interval two we have 1.3 = distance / t
and this equals to: distance = 1.3t

Then we can plug in that value as the d and solve for t :D

• Samy_A