Events & Promotions
|
|
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Sep 1507:00 PM EDT
-09:00 PM EST
Elite scores are possible when you enroll in a powerful GMAT course, taught live online + 6 months access to TTP OnDemand video courses included! Class starts Tues/Thurs Sept. 15, 2026 - Nov. 15, 2027, 7:00pm-9:00pm EST
Sep 2107:00 PM EDT
-10:00 PM EDT
Boost your GMAT score in less than one month in a live online class + 6 months access to TTP OnDemand video courses included! Class starts Mon, Tues, Wed, Thur, Fri Sept. 21, 2026 - Oct. 9, 2027, 7:00pm-10:00pm EST
30 Mar 2019, 07:36
Kudos
Bookmarks
Someone asked me a question on average speed in a different post. So, adding some theory that may help others.
Average speed = \(\frac{Distance}{time}\). We can generally solve such questions by calculating distance over the entire period and the time taken.
For example -
(I) If you are given two speeds s1 for t1 and s2 for t2, the average speed will be \(\frac{s1*t1+s1*t2}{t1+t2}\). This is nothing but \(\frac{Total ..distance}{total..time}\)
(II) If you are given two speeds s1 and s2 over a distance of d1 and d2 respectively, the average speed will be \((d1+d2)/(\frac{d1}{s1}+\frac{d2}{s2})\). This is nothing but \(\frac{Total ..distance}{total..time}\)
The problem comes in when we have to calculate the average speed from just two average speeds with no additional information. Always remember, it will not be the SUM of two speeds over 2 and would require some additional information other than the two below mentioned cases.
Speed of say 10km per hour and 20 km per hour where
1) distance is same say 20 km..
Distance is 20+20=40 and time taken is \(\frac{20}{10}+\frac{20}{20}=2+1=3\).
Average speed is 40/3, which can also be calculated from \(\frac{2*s_1*s_2}{s_1+s_2}=\frac{2*10*20}{10+20}=\frac{400}{30}=\frac{40}{30}.\)
2) time is same say 2 hours
Distance covered = \(s_1*t_1+s_2*t_2=10*2+20*2=60\) km, while time taken is 2+2=4...average speed = 60/4=15, which can also be calculated from \(\frac{s_1+s_2}{2}=\frac{10+20}{2}=15\)
So it is only in these cases that you can find the average speed from two average speed - One when time is same and second when distance is same. In almost all other cases you require some more information - Either time taken or distance traveled over these two speeds.
I will add few OG questions in the thread at a later stage
Average speed = \(\frac{Distance}{time}\). We can generally solve such questions by calculating distance over the entire period and the time taken.
For example -
(I) If you are given two speeds s1 for t1 and s2 for t2, the average speed will be \(\frac{s1*t1+s1*t2}{t1+t2}\). This is nothing but \(\frac{Total ..distance}{total..time}\)
(II) If you are given two speeds s1 and s2 over a distance of d1 and d2 respectively, the average speed will be \((d1+d2)/(\frac{d1}{s1}+\frac{d2}{s2})\). This is nothing but \(\frac{Total ..distance}{total..time}\)
The problem comes in when we have to calculate the average speed from just two average speeds with no additional information. Always remember, it will not be the SUM of two speeds over 2 and would require some additional information other than the two below mentioned cases.
Speed of say 10km per hour and 20 km per hour where
1) distance is same say 20 km..
Distance is 20+20=40 and time taken is \(\frac{20}{10}+\frac{20}{20}=2+1=3\).
Average speed is 40/3, which can also be calculated from \(\frac{2*s_1*s_2}{s_1+s_2}=\frac{2*10*20}{10+20}=\frac{400}{30}=\frac{40}{30}.\)
2) time is same say 2 hours
Distance covered = \(s_1*t_1+s_2*t_2=10*2+20*2=60\) km, while time taken is 2+2=4...average speed = 60/4=15, which can also be calculated from \(\frac{s_1+s_2}{2}=\frac{10+20}{2}=15\)
So it is only in these cases that you can find the average speed from two average speed - One when time is same and second when distance is same. In almost all other cases you require some more information - Either time taken or distance traveled over these two speeds.
I will add few OG questions in the thread at a later stage
Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Quantitative Questions Forum
Still interested in this question? Check out the "Best Topics" block below for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
