Why Did My Calculation of Driving Speed Result in an Error?

[Manodesi524]

Homework Statement

You jog at 9.8 km/ h for 8.0 km, and then you jump in the car and drive an additional 16km. With what average speed must you drive the car if your avg speed for the entire 24 km is to be 22km/ h

Homework Equations

Average speed = distance/ time

The Attempt at a Solution

I found the time it took for my jog, which was approx. .84 miles.
Then I made an equation using some facts :

22= 24/ .84+ x

X is about .25 hours

So now that I know this I plug it into my equation that will solve the problem: 16/.25

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?

 

[Mark M]

You made an error calculating x. If 22 = 24 / (.84 + x), we know that the denominator for that equation must be equal to 24 / 22. Calculate that, then subtract .84 to get the time in the car. Then go from there.

 

[Manodesi524]

Mark, i am sure x is .25. I've done the calculations several times. I would like an answer soon because today's test is modeled after the homework

 

[NascentOxygen]

I found the time it took for my jog, which was approx. .84 miles.

0.8163 hrs

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?

I say it's about midway between those values. Your method is correct, just need to retain more sig figs.

 

[SammyS]

Homework Statement

You jog at 9.8 km/ h for 8.0 km, and then you jump in the car and drive an additional 16km. With what average speed must you drive the car if your avg speed for the entire 24 km is to be 22km/ h

Homework Equations

Average speed = distance/ time

The Attempt at a Solution

I found the time it took for my jog, which was approx. .84 miles.
Then I made an equation using some facts :

22= 24/ .84+ x

X is about .25 hours

So now that I know this I plug it into my equation that will solve the problem: 16/.25

My final answer is about 64 km/ hr.

My teacher says its 50. Where did I go wrong?

Why would you use miles to solve any part of this problem?