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

Redeem

Algebra

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Moderator
Posts: 772
Joined: Wed Aug 30, 2017 6:29 pm
Followed by:6 members

Algebra

by BTGmoderatorRO » Sat Oct 28, 2017 3:21 am
It takes 30 days to fill a laboratory dish with bacteria. If the size of the bacteria doubles each day, how long did it take for the bacteria to fill one half of dish?

A) 10 days
B) 15 days
C) 24 days
D) 29 days
E) 29.5 days
OA is d
It's a simple problem here but I am struggling to express what happens here algebraically. Please help me :? :?
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 555
Joined: Wed Oct 04, 2017 4:18 pm
Thanked: 180 times
Followed by:12 members

by EconomistGMATTutor » Sat Oct 28, 2017 3:44 am
Hello Roland. I hope I can clarify it to you.

You can establish a function that represents the growth of bacteria as follows:
$$f\left(t\right)=2^t$$
where t is the number of days.

If it takes 30 days to fill a laboratory dish it implies that the capacity of the laboratory dish is $$f\left(30\right)=2^{30}.$$

The half of the capacity is $$\frac{2^{30}}{2}=2^{29}.$$

Note that this final result is equal to f(29).

So, the half of the laboratory dish is filled after 29 days,

The correct answer is D.

I'm available if you'd like any follow up.

Regards.
GMAT Prep From The Economist
We offer 70+ point score improvement money back guarantee.
Our average student improves 98 points.

Image
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 8086
Joined: Sat Apr 25, 2015 10:56 am
Location: Los Angeles, CA
Thanked: 43 times
Followed by:29 members

by Scott@TargetTestPrep » Tue Nov 12, 2019 7:18 pm
BTGmoderatorRO wrote:It takes 30 days to fill a laboratory dish with bacteria. If the size of the bacteria doubles each day, how long did it take for the bacteria to fill one half of dish?

A) 10 days
B) 15 days
C) 24 days
D) 29 days
E) 29.5 days
OA is d
It's a simple problem here but I am struggling to express what happens here algebraically. Please help me :? :?
Since the size of the bacteria doubles each day and it takes 30 days to fill the laboratory dish with bacteria, it must be true that it took 29 days to fill half of the dish with bacteria (since the following day, the 30th day, the size of the bacteria will double and fill the entire dish).

Answer: D

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
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Wed Nov 13, 2019 7:07 am
BTGmoderatorRO wrote:It takes 30 days to fill a laboratory dish with bacteria. If the size of the bacteria doubles each day, how long did it take for the bacteria to fill one half of dish?

A) 10 days
B) 15 days
C) 24 days
D) 29 days
E) 29.5 days
KEY INFO: The size of the bacteria doubles each day
So, we can write: (size of bacteria on Day 29)(2) = (size of bacteria on Day 30)

GIVEN: On Day 30, the dish is at 100% capacity
So, we can write: (size of bacteria on Day 29)(2) = (100% capacity)
Divide both sides by 2 to get: (size of bacteria on Day 29) = (100% capacity)/2
Simplify: (size of bacteria on Day 29) = 50% capacity

So, on Day 29, the dish was HALF (aka 50%) full.

Answer: D

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Post new topic Post Reply
• Page 1 of 1