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The present ratio of students to teachers at a certain

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Intern
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Joined: 21 Sep 2016
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GMAT 1: 430 Q38 V13
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Re: The present ratio of students to teachers at a certain  [#permalink]

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New post 27 Mar 2019, 00:52
@KarishmaVeritasPrep Hi, can u please help me explain the above question using weighted average concept..
Thanks in advance. :)
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New post 27 Mar 2019, 00:55
Hi VeritasKarishma , can u please explain this question using weighted average concept.It will b a great help,thank u in advance.:)

I'm stuck solving it through weighted average :(
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New post 02 Jan 2020, 21:45
Hi VeritasKarishma

Why can't we use an approach similar to like this used on another ratio related question ?

\(https://gmatclub.com/forum/the-number-of-stamps-that-kaye-and-alberto-had-were-in-the-98116.html#p1640044\)
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The present ratio of students to teachers at a certain  [#permalink]

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New post 03 Jan 2020, 00:01
Impenetrable wrote:
The present ratio of students to teachers at a certain school is 30 to 1. If the student enrollment were to increase by 50 students and the number of teachers were to increase by 5, the ratio of students to teachers would then be 25 to 1. What is the present number of teachers?

(A) 5
(B) 8
(C) 10
(D) 12
(E) 15

This might be really embarrasing, since I normally score Q47 or higher, but this question really made me stumble.

What is wrong with my approach (I always use this approach on ratio questions and it normally works well):

Plug in E:

25:1 is the new ratio -> 1 = 1*15 = 15 teachers and 25 = 25*15 = 375 students

15 - 5 = 10 teachers before the increase
375 - 50 = 325 students before the increse

so the old ratio is 325 : 10 which is unequal to 30:1

Where is my mistake?

PS. No need to tell me others ways how to solve it. I am familiar with the 'equations ways'.


Cheers a lot and I am happy ti give away Kudos ;)
Lars


Saumya2403:

You can use weighted average here but the figures are not conducive to that method.

A1 = 1/31 - Concentration of teachers in the group
A2 = 5/55 = 1/11 - Concentration of teachers in the added group
Aavg = 1/26 - Concentration of teachers in the combined group

w1/w2 = (1/11 - 1/26) / (1/26 - 1/31) = 15*31/5*11 = 93/11

Ratio of total group earlier : group added = 93 : 11 = x : 55
So there were 465 people earlier out of which 1/31 were teachers i.e. (1/31)*465 = 15 teachers

Answer (E)
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Re: The present ratio of students to teachers at a certain  [#permalink]

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New post 03 Jan 2020, 00:06
jabhatta@umail.iu.edu wrote:
Hi VeritasKarishma

Why can't we use an approach similar to like this used on another ratio related question ?

\(https://gmatclub.com/forum/the-number-of-stamps-that-kaye-and-alberto-had-were-in-the-98116.html#p1640044\)


We can't use the LCM method here because the number in the group is different in the two cases. Teachers and students were added. So it is like 2 mixtures were added to give an avg.
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Re: The present ratio of students to teachers at a certain   [#permalink] 03 Jan 2020, 00:06

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