If the population of a certain country increases...

BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

This topic has expert replies

Post new topic Post Reply

Previous Topic Next Topic

swerve: Legendary Member; Posts: 2499; Joined: Sun Oct 29, 2017 2:04 pm; Followed by:6 members

If the population of a certain country increases...

by swerve » Mon Jan 08, 2018 6:47 am

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.

Quote

Source: Beat The GMAT — Problem Solving | Mon Jan 08, 2018 6:47 am

GMAT/MBA Expert

ErikaPrepScholar: 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 $$20\ \ \cdot\ \left(\frac{4}{1\ }\right)\cdot\left(\frac{1\ person}{1\ }\right) = 80\ people$$

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

Quote

GMAT/MBA Expert

Scott@TargetTestPrep: GMAT Instructor; Posts: 8085; 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]