# How do I solve these problems (kinematics)

Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip

The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So whats the proper way im suppose to solve this?

SteamKing
Staff Emeritus
Homework Helper
Homework Statement
She drives for 40.0 min at 80.0 km/h, 10.0 min at 75.0 km/h, and 30.0 min at 45.0 km/h

Determine the average speed for the trip

The attempt at a solution
I got 66.25khm by ((4*80)+(75)+(3*45))/8 which looks like its in the ballpark range but when I enter the answer for my online homework its incorrect.

So whats the proper way im suppose to solve this?
What fraction of an hour is 10 minutes?

I appreciate the answer the question but I would enjoy more on understanding how to do them myself

Other questions on my online homework I don't get
Two boats start together and race across a 70-km-wide lake and back. Boat A goes across at 70 km/h and returns at 70 km/h. Boat B goes across at 35 km/h, and its crew, realizing how far behind it is getting, returns at 105 km/h. Turnaround times are negligible, and the boat that completes the round trip first wins.

Boat A wins but by how much? What is the average velocity of the winning boat?
since the boat starts moving at 70km/h and returns at 70km/h how come its says im wrongs when I enter 70?

What fraction of an hour is 10 minutes?
1/6th

SteamKing
Staff Emeritus
Homework Helper
1/6th
Why did you divide your calculation in Post #1 by 8 then?

Why did you divide your calculation in Post #1 by 8 then?

because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.

SteamKing
Staff Emeritus
Homework Helper
because they traveled for 80 minutes

I don/t know the proper equation to punch these numbers into.
The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.

The basic equation to use is distance = rate × time

The only thing I can see that would be wrong with your original answer of 66.25 kph is that this figure has more significant figures than the original data. If you expressed it to 3 significant figures, that might be the difference between a correct result and an erroneous one.
but my answer is correct right?

SteamKing
Staff Emeritus