Appreciate your Effort but dear you are wrong. Ans of this question is 3.33 Hours (given in the book from which i have copied this question)

My test center is PUNJAB COLLEGE OF SCIENCE FOR BOYS, P-404, TEZAB MILL CHOWK, JARANWALA ROAD, FAISALABAD.
Fouzia, i hope our roll # will be on same room
Best of Luck dear.

According to formula:
if A can do a piece of work in x hours and B can do it in y hours then A and B working together will do the same work in xy/x+y hours.

here, let A work alone in , X= 5 hours
let B work alone in , Y hours
Both work together in = 2 hours
put in formula...
(5)(y)/5+y = 2 hours
5y/ 5+y= 2
5y = 2 (5+y)
5y = 10+ 2y
5y-2y = 10
3y = 10
y = 10/3 = 3.333
So, B can work alone in 3.33 hours.

That formula is given in my book.

if anyone has easy method please share

__________________
"Hope for the Best"

if A and B do a job together in 2 hours, then in one hour, 1/2 will get done.
In general, if a job takes x hours, then in one hour, 1/x will get done.
So, Let x be how long will it take B to do the same job alone.
Then
1/x is that fraction of the job that gets done in one hour.
Therefore,
A and B do a job together in 2 hour = 1/2 of the job
and
A do the same job in 5 hours = 1/5 of the job
.
Wait a second
here was a little mistake

We have,
1/5+1/x = 1/2
1/x = 1/2 -1/5
1/x = 5-2/10
1/x = 3/10
Therefore, on taking reciprocals,
x = 10/3 hours (3.33 hours)

Sorry for my mistake and thnx for correcting me..!!

__________________
If you don't have option(s), you are an option.

Problem 1. Half of a number added to its third part, is eight less than the number. What is the number?

Problem 2. Three fifths of a number, plus 8, is equal to the number. What is the number?

Problem 3. Half of a number, plus a fifth of two less than the number, is four less than the number. What is the number?

Problem 4. Melissa went shopping and spent half of her money on shoes, a third on a blouse, a tenth to take her boyfriend to lunch, and she came home with $12. How much did she start out with?


Problem 5. One number is four times another. The sum of their reciprocals is 5/12. What are the two numbers?

Problem 6 If the same number is added to the numerator and denominator of 7/9 the result is 5/6. What is that number?

Problem 7. The speed of a boat in still water is 15 mi/hr. If the boat travels 8 miles downstream in the same time it takes to travel 4 miles upstream, what is the speed of the current?

Problem 8. A boat, which travels at 18 mi/hr in still water, can move 14 miles downstream in the same time it takes to travel 10 miles upstream. Find the speed of the current.

Problem 9. Train A has a speed 15 mi/hr greater than train B. If train A travels 150 miles in the same time train B travels 120 miles, what are the speeds of the two trains?

Problem 10. A train travels 30 mi/hr faster than a car. If the train covers 120 miles in the same time the car covers 80 miles, what is the speed of each of them?

Problem 11. You have exactly h hours at your disposal. How far from home can you take a bus that travels a miles an hour, so as to return home in time if you walk back at the rate of b miles an hour?

Problem 12. Carlos can do a certain job in three days, while it takes Alec six days. If they work together, how long will it take them?

Problem 13. If Jane can do a certain job in 6 hours, but it takes Ana only 4 hours, how long will it take them if they work together?

Problem 14. Two people working together can complete a job in six hours. If one of them works twice as fast as the other, how long would it take the faster one working alone?

__________________
If you don't have option(s), you are an option.

Problem 1
let X be the Number
1/2 x+1/3 x=x-8
3x+2x=6x-48
5x=6x-48
x=48

Problem 2

3/5 x +8 = x ( multiply both sides with 5)
3x+40=5x
40=2x
x=20

Problem 3
1/2 x+1/5(x-2)= x-4
1/2 x+x/5 - 2/5=x-4
(5x+2x/10) - 2/5 = x-4
7x/10-2/5=x-4
(7x-4/10)=x-4
7x=10x-40+4
3x=36
x=12

Easy method:
A can do it in 5 hours; therefore he can do 1/5 in one hour. The total time is 2 hours; so both A & B can do ½ in one hour.
1/2-1/5= 5-2/10=3/10
B can do 3/10 in one hour; so he can do the the same job in 10/3 hours.(reciprocal of 3/10).

Note:
*unit time is expressed in terms of 1 min, 1 hour, 1 day etc.
*The rate at witch a person works is the amount of time he can do in unit time.
*If given the time it will take one person to do a job; then the reciprocal of the time is the part done in unit time.

__________________
Aalam-e-soz o saz main, wasl se barh ker hai firaaq
Hijr me lazt-e-talb, wasl main marg-e-arzoo...!!!

Its quite easy, but the point which is missing is this that we need to make an equation while solving such type of problems.. like in this case this need to be understand that all the work should be expressed in same unit of time as mentioned by sibgakhan...consider following while solving this question

in one hour:
A can do 1/5 of work
A and B can do 1/2 of work
and B can do 1/x of work

making equation of above data we get

A + B = work done by both of them
(1/5) + (1/x) = 1/2

Now solve the equation for x.

__________________
Regards,
~ Riz ~

60*9*1000/3600

150

__________________
To escape criticism - do nothing, say nothing, be nothing.

A train 125 m long passes a man, running at 5 km/hr in the same direction in which the train is going, in 10 seconds. The speed of the train is?

__________________
"The future depends on what we do in the present"