Problem Solving (PS) | Re: Mrs. Jordan visited a certain city. She travelled the entire distance

Can't we use mixtures formula here? w1/w2 = a2-abg/avg-a1, the answer comes out as 35 this way
My response:
We cannot use the mixture formula directly because that will give Arithmetic Mean (A.M) whereas the average of two speeds will be Harmonic mean (HM) ( Average speed ≠ Average of two speeds). However we can use the weighted average formula along with the concept that AM > HM. So let's take the speed for 1/3 distance as x.
Then AM= (2/3*50+1/3*x)/(2/3+1/3)=45(given) => x=35. However, actually as per the problem, HM=45
Since AM> HM; AM > 45
IF AM > 45, than x > 35. Look into answer choice for the value which is just above 35. So choose 37.5
Ans C This trick will always work in GMAT­

Statistics : Posted by KB1920 • on 01 Mar 2019, 00:52 • Replies 7 • Views 6878

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