the value of T is

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sanju09: GMAT Instructor; Posts: 3650; Joined: Wed Jan 21, 2009 4:27 amFollowed by: 80 members; Location: India; Thanked: 267 times; GMAT Score:760

by sanju09 » Sat Oct 22, 2011 3:47 am

T varies directly as the square of r and inversely as the cube of s. If r is tripled and s is doubled, the value of T is
(A) multiplied by 3/2
(B) multiplied by 6
(C) multiplied by 9/8
(D) multiplied by 2
(E) divided by 2

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Source: Beat The GMAT — Problem Solving | Sat Oct 22, 2011 3:47 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sat Oct 22, 2011 4:03 am

sanju09 wrote:T varies directly as the square of r and inversely as the cube of s. If r is tripled and s is doubled, the value of T is

T is directly proportional to square of r.
Hence, if r becomes 3r, T will become (3Â²)T = 9T

T is inversely proportional to cube of s.
Hence, if s becomes 2s, T will become T/(2Â³) = T/8

Hence, overall T will become (9/8)T
[spoiler]
The correct answer is C.[/spoiler]

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GmatKiss: Legendary Member; Posts: 2789; Joined: Tue Jul 26, 2011 12:19 amFollowed by: 43 members; Location: Chennai, India; Thanked: 206 times; GMAT Score:640

by GmatKiss » Sat Oct 22, 2011 7:31 am

Not getting it

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rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 amFollowed by: 9 members; Location: New Delhi, India; Thanked: 130 times; GMAT Score:720

by rijul007 » Sat Oct 22, 2011 10:19 am

T varies directly as the square of r and inverely as a cube of s

T = k(r^2)/(s^3)

value of T when r is tripled and s is double = T'

T' = k((3r)^2)/((2s)^3) = k(9r^2)/(8s^3) = k * 9/8 * (r^2)/(s^3) = 9/8 * T

Hence C is the correct option.

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zooki: Junior | Next Rank: 30 Posts; Posts: 29; Joined: Fri Oct 14, 2011 10:15 amFollowed by: 1 member; Thanked: 4 times

by zooki » Sat Oct 22, 2011 11:00 am

T is directly proportional to square if r
i. e. T âˆž r^2
T is Inversly proportional to cube of s
i. e. T âˆž 1/s^3

if r triples we get T âˆž (3r)^2= 3^2*r^2= 9r^2
if s doubles we get T âˆž 1/(2s)^3= 1/(2^3*s^3)= 1/(8s^3)

Therefore, if the given conditions happen, the value of T becomes (9/8)T

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