Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 06 Jul 2015, 23:23

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# x percent of y percent of z is decreased by y percent. What

Author Message
TAGS:
Intern
Joined: 20 Feb 2012
Posts: 11
Followers: 0

Kudos [?]: 1 [0], given: 0

x percent of y percent of z is decreased by y percent. What [#permalink]  13 Apr 2012, 14:56
00:00

Difficulty:

35% (medium)

Question Stats:

75% (01:47) correct 25% (01:04) wrong based on 16 sessions
x percent of y percent of z is decreased by y percent. What is the result?

A. (100xyz - xy^2z) / 1,000,000
B. (xz - y) / 100
C. (xz - y) / 10,000
D. (xyz - 2y) / 100
E. (xyz - 2y) / 10,000

[Reveal] Spoiler:
A

I don't understand on these types of problems when to solve by direct algebra or "picking numbers" as the MGMAT book says. I calculated this probably like this:

( (x/100) * (y/100) * z ) - (y/100) = (xyz/10,000) - (y/100) = (xyz - 100y) / 10,000

MGMAT picks numbers out of thin air which I'm not the greatest at. Is there anyway to solve this with direct math?
[Reveal] Spoiler: OA
 Manhattan GMAT Discount Codes Kaplan GMAT Prep Discount Codes GMAT Pill GMAT Discount Codes
Math Expert
Joined: 02 Sep 2009
Posts: 28331
Followers: 4484

Kudos [?]: 45394 [0], given: 6759

Re: Percent of x percent of y... type question [#permalink]  13 Apr 2012, 15:01
Expert's post
RyanP wrote:
X percent of Y percent of Z is decreased by Y percent. What is the result?

A) (100xyz - xy^2Z) / 1,000,000

B) (XZ-Y) / 100

C) (XZ-Y) / 10,000

D) (XYZ - 2Y) / 100

E) (XYZ - 2Y) / 10,000

[Reveal] Spoiler:
A

I don't understand on these types of problems when to solve by direct algebra or "picking numbers" as the MGMAT book says. I calculated this probably like this:

( (x/100) * (y/100) * z ) - (y/100) = (xyz/10,000) - (y/100) = (xyz - 100y) / 10,000

MGMAT picks numbers out of thin air which I'm not the greatest at. Is there anyway to solve this with direct math?

x percent of y percent of z is decreased by y percent. What is the result?

Step by step:
1. $$y$$ percent of $$z$$ is $$\frac{y}{100}*z=\frac{yz}{100}$$;
2. $$x$$ percent of above value is $$\frac{x}{100}*\frac{yz}{100}=\frac{xyz}{10,000}$$;
3. The above value is decreased by $$y$$ percent: $$\frac{xyz}{10,000}*(1-\frac{y}{100})=\frac{xyz}{10,000}*(\frac{100-y}{100})=\frac{100xyz-xy^2z}{1,000,000}$$ (for example 100 is decreased by 10 percent can be expressed as $$100*(1-\frac{10}{100})=100*(1-0.1)=100*0.9=90$$).

Hope it's clear.
_________________
Intern
Joined: 20 Feb 2012
Posts: 11
Followers: 0

Kudos [?]: 1 [0], given: 0

Re: x percent of y percent of z is decreased by y percent. What [#permalink]  13 Apr 2012, 15:05
I see the process you went through but I'm still not sure I understand it. How do you know to multiply (1-y/100) instead of subtracting it? The word decrease makes me automatically think it has to be subtracted.

In this types of problems, are you only going to add/subtract if it says greater than or less than?
Math Expert
Joined: 02 Sep 2009
Posts: 28331
Followers: 4484

Kudos [?]: 45394 [0], given: 6759

Re: x percent of y percent of z is decreased by y percent. What [#permalink]  13 Apr 2012, 15:08
Expert's post
RyanP wrote:
I see the process you went through but I'm still not sure I understand it. How do you know to multiply (1-y/100) instead of subtracting it? The word decrease makes me automatically think it has to be subtracted.

In this types of problems, are you only going to add/subtract if it says greater than or less than?

The question does not say "decreased by y", it says "decreased y PERCENT".
_________________
Re: x percent of y percent of z is decreased by y percent. What   [#permalink] 13 Apr 2012, 15:08
Similar topics Replies Last post
Similar
Topics:
2 X is what percent greater than Y percent of Z? 4 25 Apr 2012, 23:42
10 x percent of y percent of z is decreased by y percent. What 10 23 Sep 2011, 00:05
x is what percent of y percent of z, in terms of x, y and z? 5 28 Oct 2010, 12:51
x is what percent cheaper than y 2 04 Jul 2010, 00:13
14 X percents of the rooms are suits, Y percent of the rooms 12 17 Oct 2007, 22:24
Display posts from previous: Sort by