# average speed

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• Nov 18th 2012, 04:07 AM
rcs
average speed
Tom drove for 3 hours at 30 km/hr and another 2 hrs at 70km/h. What was his average speed for the journey?

attempt:

using S = d/t

Given: 2 hrs at 70km/h and 3 hrs at 30km/h

Solve: 30/3 + 70/2 = (10 + 35 ) /2 = 45/2 = 22.5 km/h

please correct me if im wrong.
because im not so sure with my solution.

thanks
• Nov 18th 2012, 05:03 AM
BobP
Re: average speed
Yes, you are wrong.
How far does he travel in the first 3 hours ?
How far does he travel in the next 2 hours ?
What is his total distance travelled ?
What is the total time taken ?
What is his average speed ?
• Nov 18th 2012, 05:28 AM
rcs
Re: average speed
Quote:

Originally Posted by BobP
Yes, you are wrong.
How far does he travel in the first 3 hours ?
How far does he travel in the next 2 hours ?
What is his total distance travelled ?
What is the total time taken ?
What is his average speed ?

What was his average speed for the journey?

is only ask... but may i know how it is solved ...
thanks
• Nov 18th 2012, 05:31 AM
a tutor
Re: average speed
Answer Bob's questions and you will have your answer.
• Nov 18th 2012, 05:33 AM
rcs
Re: average speed
3(30) + 2(70) = 90 + 140 = 230 km total
5 hrs also in total

230/5 = 46

so it is 46 km/hr is the answer..

can anyone check my solution because im so sure with my answer

thanks
• Nov 18th 2012, 05:35 AM
HallsofIvy
Re: average speed
Quote:

Originally Posted by rcs
What was his average speed for the journey?

is only ask... but may i know how it is solved ...
thanks

By using the formula you wrote in your first post- average speed is the distance traveled divided by the time. And those are given by answering the questions in the first response.
• Nov 18th 2012, 06:06 AM
rcs
Re: average speed
how come and how it is really done?
• Nov 18th 2012, 08:40 AM
topsquark
Re: average speed
Quote:

Originally Posted by rcs
how come and how it is really done?

The equation for average speed is based on the total distance over the total time. So $\displaystyle v_{ave} = \frac{\text{total distance}}{\text{total time}} = \frac{d}{t}$

-Dan
• Nov 18th 2012, 05:19 PM
Prove It
Re: average speed
Quote:

Originally Posted by rcs
3(30) + 2(70) = 90 + 140 = 230 km total
5 hrs also in total

230/5 = 46

so it is 46 km/hr is the answer..

can anyone check my solution because im so sure with my answer

thanks

This is correct.