19-02-2013, 02:33 PM
IMPORTANT FACTS AND FORMULA
IMPORTANT FACTS.pdf (Size: 158.9 KB / Downloads: 178)
SOLVED EXAMPLES
Ex. 1. Worker A takes 8 hours to do a job. Worker B takes 10 hours to do the same Job.How
long should it take both A and B, working together but independently, to do the
same job? (IGNOU, 2003)
Sol. A’s 1 hour's work = 1/8
B's 1 hour's work = 1/10
(A + B)'s 1 hour's work = (1/8) +(1/10)=9/40
Both A and B will finish the work in 40/9 days.
Ex. 2. A and B together can complete a piece of work in 4 days. If A alone can complete the
same work in 12 days, in how many days can B alone complete that work? (Bank P.O.
2003)
Sol. (A + B)'s 1 day's work = (1/4). A's 1 day's work = (1/12).
B's 1 day's work =((1/4)-(1/12))=(1/6)
Hence, B alone can complete the work in 6 days.
Ex. 3. A can do a piece of work in 7 days of 9 hours each and B can do it in 6 days of 7 bours each. How long will they take to do it, working together 8 hours a day?
Sol. A can complete the work in (7 x 9) = 63 hours.
B can complete the work in (6 x 7) = 42 hours.
A’s 1 hour's work = (1/63) and B's 1 hour's work =(1/42)
(A + B)'s 1 hour's work =(1/63)+(1/42)=(5/126)
Both will finish the work in (126/5) hrs.
Number of days. of (42/5) hrs each =(126 x 5)/(5 x 42)=3 days
Ex. 4. A and B can do a piece of work in 18 days; Band C can do it in 24 days A and C can do
it in 36 days. In how many days will A, Band C finish it together and separately?
Sol. (A + B)'s 1 day's work = (1/18) (B + C)'s 1 day's work = (1/24)
and (A + C)'s 1 day's work = (1/36)
Adding, we get: 2 (A + B + C)'s 1 day's work =(1/18 + 1/24 + 1/36)
=9/72 =1/8
(A +B + C)'s 1 day's work =1/16
Thus, A, Band C together can finish the work in 16 days.
Now, A’s 1 day's work = [(A + B + C)'s 1 day's work] - [(B + C)'s 1 day work:
=(1/16 – 1/24)= 1/48
A alone can finish the work in 48 days.
Similarly, B's 1 day's work =(1/16 – 1/36)=5/144
B alone can finish the work in 144/5=28 4/5 days
And C’s 1 day work =(1/16-1/18)=1/144
Hence C alone can finish the work in 144 days.