docabuzar wrote:
Tom, working alone, can paint a room in 6 hours. Peter and John, working independently, can paint the same room in 3 hours and 2 hours, respectively. Tom starts painting the room and works on his own for one hour. He is then joined by Peter and they work together for an hour. Finally, John joins them and the three of them work together to finish the room, each one working at his respective rate. What fraction of the whole job was done by Peter?
A. 1/9
B. 1/6
C. 1/3
D. 7/18
E. 4/9
First Hr :
T starts working and in 1 hour can finish 1/6 of the job
Second Hr:
T & P starts working and in an hr can finish 1/6+1/3 = 3/6 of the job. So Total 4/6 of the job is finished by now
Third Hr:
T,P & J starts working but they have only 2/6 of the job remaining.
Working together they need one hr to finish the entire job (work formula 1/6+1/3+1/2 = 1/1 = 1 hr)
so they work only for 2/6 of an hour.
THerefore peter working at a rate of 1/3 can do only 1/3*2/6 = 1/9 of the job before the job is finished.
Total Job done by Peter = 1/3+1/9 = 4/9
Is there a shorter or quicker way to do it?
Lets try to solve this question without complicated algebra, with just very easy to compute numbers.
If the question permits, we should always try to use multiples of 10, to substitute any numbers to establish the relations in the question.
Lets consider the total work to paint the room = 60 units.
Given Tom alone can paint the room in 6 hours, Work Rate of Tom = 60/6 = 10 units/hour
Peter alone can paint the room in 3 hours, Work Rate of Peter = 60/3 = 20 units/hour
John Alone can paint the room in 2 hours, Work Rate of John = 60/2 = 30 units/hour
Tom starts the work & works alone for 1 hour, hence Tom finishes 10 units of the work.
Work left to finish = 60 - 10 = 50 units of work
Now Peter joins Tom & they together Work for an hour, hence Tom & Peter finish (10 + 20) = 30 units of work
Work left to finish now is = 50 - 30 = 20 units of work
Lastly, John joins Tom & Peter & the three work together to finish the job.
The three together can finish (10 + 20 + 30) = 60 units of work in one hour or 60 minutes.
So working together the rate at which the work is done is (60 units/60 minutes) = 1 unit/min.
Hence to finish the 20 units of work that is left, the three together will take 20 mins.
The question asks for the fraction of total work, as done by Peter.
Peter worked for a total of 1 hour & 20 mins. Hence total units of work done by peter = Work done in 1 hour + Work Done in 20 mins
Work Done by Peter in 60 mins = 20 units
Work Done by Peter in 20 mins = (20 * 20)/60 = 20/3 units of work
Total work done by Peter = 20 + 20/3 = 80/3 units of work.
Hence Peters work, as a fraction of Total Work = (80/3) / 60 = 80/(3*60) = 8/18 = 4/9
Answer E.