If the population of a certain country increases...

This topic has expert replies
Legendary Member
Posts: 2218
Joined: Sun Oct 29, 2017 2:04 pm
Followed by:6 members
If the population of a certain country increases at the rate of one person every 15 seconds, by how many persons does the population increase in 20 minutes?

A. 80
B. 100
C. 150
D. 240
E. 300

The OA is A.

Please, can any expert explain this PS question for me? I tried to solve it of the following way,

I know that every 15 seconds the population increases at a rate of one person, that's mean 4 persons per minute, then in 20 minutes will be 20 x 4 = 80, right? That's it? Thanks.

GMAT/MBA Expert

User avatar
Legendary Member
Posts: 503
Joined: Thu Jul 20, 2017 9:03 am
Thanked: 86 times
Followed by:15 members
GMAT Score:770

by ErikaPrepScholar » Mon Jan 08, 2018 6:54 am
You nailed it! This problem is deceptively simple. We can also do this a more traditional way by cancelling units using conversion factors:
$$20\ \min\ \cdot\ \left(\frac{60\ \sec}{1\ \min}\right)\cdot\left(\frac{1\ person}{15\ \sec}\right)$$ Then cancel units $$20\ \ \cdot\ \left(\frac{60\ }{1\ }\right)\cdot\left(\frac{1\ person}{15\ }\right)$$ Cancel 15 from the numerator and denominator and solve $$20\ \ \cdot\ \left(\frac{4}{1\ }\right)\cdot\left(\frac{1\ person}{1\ }\right) = 80\ people$$
Image

Erika John - Content Manager/Lead Instructor
https://gmat.prepscholar.com/gmat/s/

Get tutoring from me or another PrepScholar GMAT expert: https://gmat.prepscholar.com/gmat/s/tutoring/

Learn about our exclusive savings for BTG members (up to 25% off) and our 5 day free trial

Check out our PrepScholar GMAT YouTube channel, and read our expert guides on the PrepScholar GMAT blog

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 7223
Joined: Sat Apr 25, 2015 10:56 am
Location: Los Angeles, CA
Thanked: 43 times
Followed by:29 members

by Scott@TargetTestPrep » Mon Aug 12, 2019 10:42 am
swerve wrote:If the population of a certain country increases at the rate of one person every 15 seconds, by how many persons does the population increase in 20 minutes?

A. 80
B. 100
C. 150
D. 240
E. 300
We are given that a population increases by 1 person every 15 seconds, and we need to determine how many people it will increase by in 20 minutes, which is 20 x 60 = 1200 seconds. We can let x = the number of people being added to the population in 20 minutes and create the following proportion:

1/15 = x/1200

15x = 1200

x = 80

Alternate Solution:

A rate of one person every 15 seconds equals a rate of 4 people per minute. Thus, in 20 minutes, the population is increased by 4 x 20 = 80 people.

Answer: A

Scott Woodbury-Stewart
Founder and CEO
[email protected]

Image

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews

ImageImage