How do I solve these problems (kinematics)

[carrollk9]

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 what's the proper way I am suppose to solve this?

 

[SteamKing]

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 what's the proper way I am suppose to solve this?

What fraction of an hour is 10 minutes?

 

[carrollk9]

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 I am wrongs when I enter 70?

 

[carrollk9]

What fraction of an hour is 10 minutes?

1/6th

 

[SteamKing]

1/6th

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

 

[carrollk9]

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]

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.

 

[carrollk9]

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]

but my answer is correct right?

Yes, except for the sig figs.