GMAT Prep - Mixtures - percentages

Expert replies

tutonaranjo: Senior | Next Rank: 100 Posts; Posts: 42; Joined: Wed Jun 20, 2007 8:45 am; Thanked: 1 times

by tutonaranjo » Thu Aug 30, 2007 11:37 am

The rate of a certain chemical reaction is directly proportional to the square of the concentration of chemical A and inversely proportional to the concentration of chemical B present. If the concentration of chemical B is increased 100%, which of the following is closest to the percent change in the concentration of chemical A required to keep the reaction rate unchanged?

a) 100% decrease
b) 50% decrese
c) 40% decrease
d) 40% increase
e) 50% increase

Answer is D.

Can someone help me out with an equation and a picking numbers scenario?

Source: Beat The GMAT — Problem Solving | Thu Aug 30, 2007 11:37 am

ankanas: Junior | Next Rank: 30 Posts; Posts: 20; Joined: Thu May 24, 2007 3:44 pm

by ankanas » Fri Aug 31, 2007 5:57 am

Let
A1=original concentration of A
B1= Original concentration of B

Chemical Reaction (CR1)=k*A1^2/B1

Let the new Chemical Reaction be CR2
Let the new Concentration of Chemical B be B2
A2=New concentration of Chemical A
Acc to the question
B2=2B1 (100% increase)

Therefore to keep the reaction rate unchanged CR1=CR2

K*A1^2/B1=K*A2^2/B2
Since B2=2B1

K*A1^2/B1=K*A2^2/2B1
or A1^2=A2*^2/2
or 2A1^2=A2^2
or 1.414A1=A2
Therefore 40% increase

Answer D

jaspreet.sharma: Junior | Next Rank: 30 Posts; Posts: 24; Joined: Wed Aug 22, 2007 4:49 pm; Thanked: 3 times

by jaspreet.sharma » Fri Aug 31, 2007 1:02 pm

ankanas wrote:Let
A1=original concentration of A
B1= Original concentration of B

Chemical Reaction (CR1)=k*A1^2/B1

Let the new Chemical Reaction be CR2
Let the new Concentration of Chemical B be B2
A2=New concentration of Chemical A
Acc to the question
B2=2B1 (100% increase)

Therefore to keep the reaction rate unchanged CR1=CR2

K*A1^2/B1=K*A2^2/B2
Since B2=2B1

K*A1^2/B1=K*A2^2/2B1
or A1^2=A2*^2/2
or 2A1^2=A2^2
or 1.414A1=A2
Therefore 40% increase

Answer D

Nice way to calculate it.....

resilient: Legendary Member; Posts: 789; Joined: Sun May 06, 2007 1:25 amFollowed by: 6 members; Location: Southern California, USA; Thanked: 15 times

picking numbers

by resilient » Tue Feb 05, 2008 1:34 am

my gut reaction to this is to keep the whole thing in an easier perspective and pick numbers is there a way to do that!?

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