Aptitude - Time and Work - Discussion

Discussion Forum : Time and Work - General Questions (Q.No. 7)
7.
A can do a piece of work in 4 hours; B and C together can do it in 3 hours, while A and C together can do it in 2 hours. How long will B alone take to do it?
8 hours
10 hours
12 hours
24 hours
Answer: Option
Explanation:

A's 1 hour's work = 1 ;
4

(B + C)'s 1 hour's work = 1 ;
3

(A + C)'s 1 hour's work = 1 .
2

(A + B + C)'s 1 hour's work = ( 1 + 1 ) = 7 .
4 3 12

B's 1 hour's work = ( 7 - 1 ) = 1 .
12 2 12

Therefore B alone will take 12 hours to do the work.

Discussion:
96 comments Page 7 of 10.

Himanshi mishra said:   8 years ago
@Ankita.

Follow this method.
Here total work is 12 hours as lcm of 2-4-3.
A does 3 work in 1 hour ND he finishes 12 work in four hours.
In the same way, abt A and C you know A does 3 work in 1 hour thus C also does 3 work in 1 hour so total work 6 in 1 hour thus 12 work in 2 hour is completed by A and C.

And now C does 3 work in 1 hour so b does 1 work in 1 hour and thus the both do 4 work in 1 hour and finishes in 3 hours.

As a result, B takes 1 hour to do 1 work so B Will take 12 hours to do 12 work.

Sam said:   8 years ago
You explained well @Akshay.

Xyx said:   8 years ago
How it will 7/12-1/2=1/12?
(1)

Bunny said:   8 years ago
Thank you so much @Himanshi Mishra.

I was looking for this LCM method. This is way too easy. Thanks.

Rohit said:   8 years ago
Guys!.

I simply subtracted the (A's) work - (B's+C's)work
so I got,
1/4-1/3=-1/12 for 1hour.
hence for b to do alone his work 12 hour.

Bits said:   8 years ago
A in 4hrs.

B+C _ 3.
A+C _ 2.
Total work by taking lcm of 4,3,2 is 12units.
A in one day do = 12/4= 3.
A+C_ 6.
So C=3.
In the same way,
B+C= 12/3 > 4.
So B=1.
At last B =12/1=12. That's the answer!.
(1)

Shree said:   7 years ago
Clear and detailed explanation. Thank you all.

Akshay Ladwa said:   7 years ago
A=B+C(given) ----> (1)
one day work of A+B=1/10 --> (2)
one day work of C=1/50 ---> (3).

So for total (A+B+C)'s 1 day work, we can assume from 1--> C=A-B=1/50----> (4)
So adding equation 2 and 4 we get A=3/50.

Substituting above A in equation 2 we get B=1/25.
Hence 25 days because(If A's 1 day's work =1/n then A can finish the work in n days.)).

Sukumar said:   7 years ago
Well said @Hindu.

Tejs said:   7 years ago
Thanks for your answer @Bits.


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