IMPORTANT FACTS AND FORMULAE
1. If A can do a piece of
work in n days, then A's 1 day's work = (1/n).
2.
If A’s 1 day's work = (1/n),then A can finish the work in n days.
3. A
is thrice as good a workman as B, then:
Ratio of work done by A and B = 3 :
1.
Ratio of times taken by A and B to
finish a work = 1 : 3.
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.
Ex. 6. A is twice as good a workman as B and together
they finish a piece
in 18 days. In how many
days will A alone finish the work?
Sol. (A’s 1 day’s work):)(B’s 1 days
work) = 2 : 1.
(A + B)'s 1
day's work = 1/18
Divide
1/18 in the ratio 2 : 1.
:. A’s 1 day's work =(1/18*2/3)=1/27
Hence, A alone can finish the work in
27 days.
Ex. 6.
A can do a certain job in 12 days. B is 60% more efficient than A. How many
days does B alone take
to do the same job?
Sol. Ratio of times taken by A and B =
160 : 100 = 8 : 5.
Suppose B alone takes x days to
do the job.
Then, 8 : 5 :: 12 : x = 8x = 5 x 12 =x =
7 1/2 days.
Ex. 7. A
can do a piece of work in 80 days. He works at it for 10 days B alone finishes
the remaining work in 42 days. In how much time will A and B working together,
finish the work?
Sol. Work done by A in 10
days =(1/80*10)=1/8
Remaining work = (1- 1/8)
=7/ 8
Now,7/ 8 work is done by B in 42 days.
Whole work will be done by B in (42 x 8/7) =
48 days.
A’s 1 day's
work = 1/80 and B's 1 day's work = 1/48
(A+B)'s 1
day's work = (1/80+1/48)=8/240=1/30
Hence, both
will finish the work in 30 days.
Ex. 8. A and B
undertake to do a piece of work for Rs. 600. A alone can do it in 6 days while
B alone can do it in 8 days. With the help of C, they finish it in 3 days.
!find the share of each.
Sol :C's 1 day's work = 1/3-(1/6+1/8)=24
A : B : C = Ratio
of their 1 day's work = 1/6:1/8:1/24= 4 : 3 : 1.
A’s share = Rs. (600 *4/8) =
Rs.300, B's share = Rs. (600 *3/8) = Rs. 225.
C's share = Rs. [600 - (300 + 225») = Rs. 75.
Ex. 9. A and B working separately can do a piece
of work in 9 and 12 days respectively, If they work for a day alternately, A
beginning, in how many days, the work will be completed?
(A + B)'s 2 days' work =(1/9+1/12)=7/36
Work done
in 5 pairs of days =(5*7/36)=35/36
Remaining work =(1-35/36)=1/36
On 11th day, it is A’s turn. 1/9
work is done by him in 1 day.
1/36 work is done by him
in(9*1/36)=1/4 day
Total time taken = (10 +
1/4) days = 10 1/4days.
Ex 10 .45 men can complete a work in 16 days. Six days
after they started working, 30 more men joined them. How many days will they
now take to complete the remaining work?
(45 x 16) men can complete
the work in 1 day.
1 man's 1
day's work = 1/720
45 men's 6
days' work =(1/16*6)=3/8
Remaining work =(1-3/8)=5/8
75 men's 1
day's work = 75/720=5/48
Now,5
work is done by them in 1 day.
48
5work is done by them in (48 x 5)=6 days.
8 5
8
Ex:11. 2 men and
3 boys can do a piece of work in 10 days while 3 men and 2 boys can do the same
work in 8 days.In how many days can 2 men and 1 boy do the work?
Soln: Let 1 man’s 1 day’s work =
x and 1 boy’s 1 day’s work = y.
Then, 2x+3y = 1
and 3x+2y = 1
10
8
Solving,we get: x = 7
and y = 1
200
100
(2 men + 1 boy)’s 1 day’s
work = (2 x 7 + 1 x 1 ) = 16 = 2
200 100 200
25
So, 2 men and 1 boy together can
finish the work in 25 =12 1
days
2
2
