easy question ratios

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canuckclint: Master | Next Rank: 500 Posts; Posts: 154; Joined: Tue Aug 26, 2008 12:59 pm; Location: Canada; Thanked: 4 times

easy question ratios

by canuckclint » Tue Nov 04, 2008 10:26 pm

At the beginning of the year, the ratio of juniors to seniors in high school X was 3 to 4. During the year, 10 juniors and twice as many seniors transferred to another high school, while no new students joined high school X. If, at the end of the year, the ratio of juniors to seniors was 4 to 5, how many seniors were there in high school X at the beginning of the year?

answer: 120

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Source: Beat The GMAT — Problem Solving | Tue Nov 04, 2008 10:26 pm

parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 am; Thanked: 146 times; Followed by:2 members

by parallel_chase » Tue Nov 04, 2008 11:51 pm

J/S = 3/4

4J = 3S
J = 3S/4

(J-10)/(S-20) = 4/5

5J - 50 = 4S - 80

4S - 5J = 30

4S - 15S/4 = 30

16S - 15S = 120

S =120

Hope this helps.

No rest for the Wicked....

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logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pm; Thanked: 237 times; Followed by:25 members; GMAT Score:730

Re: easy question ratios

by logitech » Tue Nov 04, 2008 11:52 pm

canuckclint wrote:At the beginning of the year, the ratio of juniors to seniors in high school X was 3 to 4. During the year, 10 juniors and twice as many seniors transferred to another high school, while no new students joined high school X. If, at the end of the year, the ratio of juniors to seniors was 4 to 5, how many seniors were there in high school X at the beginning of the year?

answer: 120

Juniors: 3X
Seniors: 4X

so you can have the 3/4 ratio

(3x-10)/(4x-20) = 4/5 ( twice as many as Juniors, 10x2=20)

X=30

Seniors: 4X : 120

LGTCH
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