Work and time 3

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

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.

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.

The OA is 6.5 days


Thanks guys

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!

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?)

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

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???

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??

i think this is repeatation of question