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In a sample of college students 40 percent are 3rd year

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Intern
Joined: 27 Jan 2005
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In a sample of college students 40 percent are 3rd year [#permalink]  03 Feb 2005, 01:05
In a sample of college students 40 percent are 3rd year students and 70 percent are not second year students. what fraction of those students who are not 3rd year students are second year students

a 3/4
b 2/3
c 4/7
d 1/2
e 3/7

I know the answer, but don't know how to get there...anyone to explain?

VP
Joined: 30 Sep 2004
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1. 40% = 3rd year students => 60% = not 3rd year students
2. 70% = not 2nd year students => 30% = 2nd year students
3. x * 60% = 30%
4. solve for x => 1/2
Director
Joined: 07 Jun 2004
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cristoph can you elaborate on this i still did not get it
Manager
Joined: 28 Jan 2005
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Making a Chart can also help, especially on the more difficult ones:

Start with what you know. 40% are third year students. 70% ARENT second year students. So 30% are Second year students and the 70
% are either first year or 3rd year. You know third year is 30%. Whats left?

%

Year 1 30

Year 2 30

Year 3 40

Then just divide the ratio- part/whole 30/60 (are NOT third year_
Manager
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the best way is to understand the concept and logic things out, WITHOUT formulas even. Thats the advice that ian777 i think gave and it works for me. Think about how many formulas u have to memorize, each one beingdifferent. Some are close versions nd u have to decide which one to use etc...its much easier to logic things out for test day. Simplify and not complicate
Intern
Joined: 04 Feb 2005
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Assume total number of students = 100

we know that 40 percent are 3rd yr students so 100*.4 = 40 3rd yr students.
this means there are 60 non-3rd yrs. (could be 1st, 2nd, or 4th)
so (# 1st yrs) + (# 2nd yrs) + (# 4th yrs) = 60

we also know that 70 are not 2nd yrs.
so (# 1yrs) + (#3rd yrs) + (# 4th yrs) = 70

but we know from the 1st statement that # 3rd yrs = 40

then the 2nd equation becomes (#1 yrs) + 40 + (# 4th yrs) = 70
---> 1st yr + 4th yrs = 30

plug this back to 1st equation
--> 30 + 2nd yrs = 60
--> 2nd yrs = 30

we want the fraction of (# 2nd yrs) / (# non 3yrd yrs ) = 30/60 = 1/2
Manager
Joined: 24 Jan 2005
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Let's take 100 students.
Then 60 is not 3rd year so denominator is 60.
No 2nd year = 100-70 = 30
So fraction = 30/60 =1/2
D   [#permalink] 04 Feb 2005, 11:38
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