Work and time 3

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eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 amFollowed by: 1 member; Thanked: 32 times

by eaakbari » Mon Apr 05, 2010 4:09 am

A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

Source: Beat The GMAT — Problem Solving | Mon Apr 05, 2010 4:09 am

ajith: Legendary Member; Posts: 1275; Joined: Thu Sep 21, 2006 11:13 pmFollowed by: 2 members; Location: Arabian Sea; Thanked: 125 times

by ajith » Mon Apr 05, 2010 4:14 am

eaakbari wrote:A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

A can finish 1/5 th of work each day and B can finish 1/10 th

A starts 1st day and finishes 1/5
B starts 2nd day and finishes 3/10 by end of the day
A starts 3nd day and finishes 5/10 by end of the day
B starts 4th day and finishes 6/10 by end of the day
A starts 5th day and finishes 8/10 by end of the day
B starts 6th day and finishes 9/10 by end of the day
A starts 7th day and finishes the work

7 is the answer

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harshavardhanc: Legendary Member; Posts: 526; Joined: Sat Feb 21, 2009 11:47 pm; Location: India; Thanked: 68 times; GMAT Score:680

by harshavardhanc » Mon Apr 05, 2010 4:38 am

eaakbari wrote:A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

one more method. If you internalize percentages concept, it will be a very strong tool for you to do these kinds of sums.

See that A does 20% of the work in 1 day and B does 10%. Working alternately, they can do 30% of the work in 2 days.

So, in 6 (2 * 3) days, they will complete 3*30% = 90% of the work.

now, you need another 10% of the work to be done, which will be done by A working for another half day.

Therefore, answer will be 6.5 days.

Regards,
Harsha

ymach3: Senior | Next Rank: 100 Posts; Posts: 51; Joined: Mon Jul 13, 2009 4:25 pm; Thanked: 1 times

by ymach3 » Wed Apr 07, 2010 4:12 am

I tried taking Work =100 books

A takes 5 days to print 100 books -> A's 1 day work=20 books per day.

B takes 10 days to print 100 book -> B's 1 day work= 10 books per day.

A starts first :

1st day - A prints -20 books.
2nd day- B prints - 10 books.
3rd day - 20 books
4th day - 10 books
5th day - 20 books
6th day - 10 books.
7th day - 20 books

at the End of the 7th day-110 books are printed.

eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 amFollowed by: 1 member; Thanked: 32 times

by eaakbari » Wed Apr 07, 2010 11:49 am

The OA is 6.5 days

Thanks guys

gmatmachoman: Legendary Member; Posts: 2326; Joined: Mon Jul 28, 2008 3:54 amFollowed by: 2 members; Thanked: 173 times; GMAT Score:710

by gmatmachoman » Wed Apr 07, 2010 12:09 pm

eaakbari wrote:A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

SO what if A & B work every third day???

Harsha bhai...plz give me some handy formula!

harshavardhanc: Legendary Member; Posts: 526; Joined: Sat Feb 21, 2009 11:47 pm; Location: India; Thanked: 68 times; GMAT Score:680

by harshavardhanc » Wed Apr 07, 2010 12:13 pm

gmatmachoman wrote:
eaakbari wrote:A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

SO what if A & B work every third day???

Harsha bhai...plz give me some handy formula!

Bhai,

do you mean : first day - A, second day-B and third day - both ( with this pattern continuing?)

Regards,
Harsha

gmatmachoman: Legendary Member; Posts: 2326; Joined: Mon Jul 28, 2008 3:54 amFollowed by: 2 members; Thanked: 173 times; GMAT Score:710

by gmatmachoman » Wed Apr 07, 2010 12:15 pm

eaakbari wrote:The OA is 6.5 days

Thanks guys

@eaakbari..

Handy formula what i could see here is : Find the total work done together and multiply by that number of days

Soln : One day work of A: 1/5
one day work of B : 1/10

Total one day work together: 3/10

Total days for working together : 10/3

Multiply this by 2( alternate days convention)

--->20/3

---> 6.5 days

We can extend if asked for 3 days/4 days/say "n" days gap.

Formula : T * n

gmatmachoman: Legendary Member; Posts: 2326; Joined: Mon Jul 28, 2008 3:54 amFollowed by: 2 members; Thanked: 173 times; GMAT Score:710

by gmatmachoman » Wed Apr 07, 2010 12:16 pm

harshavardhanc wrote:
gmatmachoman wrote:
eaakbari wrote:A can finish a work in 5 days and B can finish it in 10 days. Let A and B work every alternate day starting with A. In how many days will they finish?

Source:prep review book

SO what if A & B work every third day???

Harsha bhai...plz give me some handy formula!
Bhai,

do you mean : first day - A, second day-B and third day - both ( with this pattern continuing?)

Instead of every alternate day, if it asks every third day???

this_time_i_will: Master | Next Rank: 500 Posts; Posts: 268; Joined: Wed Mar 17, 2010 2:32 am; Thanked: 17 times

by this_time_i_will » Wed Apr 07, 2010 5:06 pm

gmatmachoman wrote:
Instead of every alternate day, if it asks every third day???

you mean A works on 1st day, then A &B take rest on 2nd day, then B works on 3rd day, then rest, then A......like this??

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pradeepkaushal9518: Legendary Member; Posts: 1309; Joined: Wed Mar 17, 2010 11:41 pmFollowed by: 5 members; Thanked: 33 times

by pradeepkaushal9518 » Wed Apr 07, 2010 11:30 pm

i think this is repeatation of question

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