It is currently 17 Nov 2017, 10:31

### 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

# If Johnny can mow the lawn in 30 minutes and with the help of his brot

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics
Author Message
TAGS:

### Hide Tags

Manager
Joined: 17 Oct 2016
Posts: 82

Kudos [?]: 37 [1], given: 17

Location: India
Concentration: General Management, Healthcare
GMAT 1: 640 Q40 V38
GMAT 2: 680 Q48 V35
GPA: 3.05
WE: Pharmaceuticals (Health Care)
If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink]

### Show Tags

10 Oct 2017, 18:12
1
KUDOS
1
This post was
BOOKMARKED
00:00

Difficulty:

15% (low)

Question Stats:

83% (00:51) correct 17% (00:50) wrong based on 36 sessions

### HideShow timer Statistics

If Johnny can mow the lawn in 30 minutes and with the help of his brother, Bobby, they can mow the lawn 20 minutes, how long would it take Bobby working alone to mow the lawn?

(A) 1/2 hour

(B) 3/4 hour

(C) 1 hour

(D) 3/2 hours

(E) 2 hours

SOURCE: NOVA GMAT
[Reveal] Spoiler: OA

_________________

Sometimes you have to burn yourself to the ground before you can rise like a phoenix from the ashes.

Dr. Pratik

Kudos [?]: 37 [1], given: 17

Math Expert
Joined: 02 Aug 2009
Posts: 5198

Kudos [?]: 5816 [0], given: 117

Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink]

### Show Tags

10 Oct 2017, 20:18
fitzpratik wrote:
If Johnny can mow the lawn in 30 minutes and with the help of his brother, Bobby, they can
mow the lawn 20 minutes, how long would it take Bobby working alone to mow the lawn?

(A) 1/2 hour

(B) 3/4 hour

(C) 1 hour

(D) 3/2 hours

(E) 2 hours

SOURCE: NOVA GMAT

hi..

two ways -

J's one minute work = $$\frac{1}{30}$$
B's and J's combined 1 minute work = $$\frac{1}{20}$$

so B's one minute work = $$\frac{1}{20}-\frac{1}{30} = \frac{3-2}{60} = \frac{1}{60}$$
so time taken will be 60 minutes or 1 hour

logical way..
J can complete the work in 30 minutes so he does 20/30 of the work in 20 minutes

the remaining work $$1-\frac{20}{30}=\frac{1}{3}$$ is done by B in 20 minute ..
so he will do comlete work in 20*3=60 minute or 1 hour

C
_________________

Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

Kudos [?]: 5816 [0], given: 117

Manager
Joined: 07 Jul 2012
Posts: 219

Kudos [?]: 64 [0], given: 65

Location: India
Concentration: Finance
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink]

### Show Tags

10 Oct 2017, 20:36
Let the work done by Bobby in 1 min=$$\frac{1}{x}$$
work done by Johnny in 1 min= $$\frac{1}{30}$$

Work done by both of them together:
$$\frac{1}{30}$$+$$\frac{1}{x}$$= $$\frac{1}{20}$$

$$\frac{1}{x}$$= $$\frac{1}{20}$$-$$\frac{1}{30}$$

=$$\frac{3-2}{60}$$= 1/60

So time taken by Bobby working alone= 60 mins or 1 hour

Kudos please if you like my explanation!
_________________

Kindly hit kudos if my post helps!

Kudos [?]: 64 [0], given: 65

Senior Manager
Joined: 22 Aug 2013
Posts: 469

Kudos [?]: 153 [0], given: 265

Location: India
Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot [#permalink]

### Show Tags

10 Oct 2017, 21:37
Let the work required in mowing the lawn = LCM of 30 & 20 = 60 units

Johnny alone does in 30 minutes, so Johnny can do 60/30 = 2 units per minute
Together, they can do in 20 minutes, so Johnny + Bobby together can do = 60/20 = 3 units per minute

Now, J does 2 units, and they both 3 units, so Bobby's per minute work = 3-2 = 1 unit

So time taken by Bobby alone = 60/1 = 60 minutes

Kudos [?]: 153 [0], given: 265

Re: If Johnny can mow the lawn in 30 minutes and with the help of his brot   [#permalink] 10 Oct 2017, 21:37
Display posts from previous: Sort by

# If Johnny can mow the lawn in 30 minutes and with the help of his brot

 new topic post reply Question banks Downloads My Bookmarks Reviews Important topics

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.