Three machines X, Y, and Z operate independently of one another.

Question

Three machines X, Y, and Z operate independently of one another. Machine X working alone can complete a job in 12 hours, machine Y working alone can complete the job in y hours, machine Z working alone can complete the job in 4 hours, and all three machines working together and independently can complete the job in hours. What is the value of y?

A) 1 3/5
B) 3
C) 3 3/7
D) 5 1/4
E) 17 3/5

A) 1 3/5
B) 3
C) 3 3/7
D) 5 1/4
E) 17 3/5

Bunuel · Answer

Please edit the highlighted part: 8/5 hours?

Thank you.

OCDianaOC · Answer

Edited! Thanks, didn't realize it didn't paste over at first.

generis · Answer

One approach,   R*T = W  : find combined rate; add rates to solve for Y's rate; and use Y's rate to find Y's time

Given 
Rates are in   \frac{Jobs}{hour}  
X's rate =   \frac{1}{12}  
Y's rate =   \frac{1}{Y}  
Z's rate =   \frac{1}{4}  
TIME all three machines take, working together, to finish the job =   1\frac{3}{5}hrs=\frac{8}{5}   hours

Combined RATE? 
  R*T = W , so  R=\frac{W}{T}  
All three, RATE =   \frac{1}{(\frac{8}{5})}=(1*\frac{5}{8})=\frac{5}{8}

Add rates, solve for Y's rate :
 ( \frac{1}{X}+\frac{1}{Y}+\frac{1}{Z})=(\frac{1}{12}+\frac{1}{Y}+\frac{1}{4})=\frac{5}{8}

(\frac{5}{8}-\frac{1}{4}-\frac{1}{12})=\frac{1}{Y}=(\frac{15}{24}-\frac{6}{24}-\frac{2}{24})=\frac{7}{24}

Y's TIME? 
  \frac{7}{24}   is Y's rate. 
The job is 1 
Rate and time are inversely proportional. Flip Y's rate fraction to get Y's time*: 
  \frac{24}{7}hrs=3\frac{3}{7}hrs

Answer C

*  R*T=W , so  T=\frac{W}{R} 
 W = 1 
 T =\frac{1}{(\frac{7}{24})}=(1 * \frac{24}{7})=(\frac{24}{7})=3\frac{3}{7} hrs

GMATBusters · Answer

The Total work can be assumed to be 120 (LCM of (time taken to complete the job) 12,4,8/5 (ignore the denominator )) =120 units
This will make the calculation quite easier.
Now No of units of works done by X per hour =  120/12 =10 
No of units of works done by Z per hour =  120/4 =30 
No of units of works done per hour by all three of them =  120/(8/5) =75 
Thus work done by Y per hour = work done by all three - (work done by X +work done by Z) =75-(10+30) =35
Hence the time taken by Y to complete the work = 120/35 = 24/7 = 3 3/7 = Answer = C

gracie · Answer

1/y= 
y=24/7=3 3/7 hours
C

nishore1 · Answer

X can complete the work in 1/12 hours
Y can complete the work in 1/ x hours
Z can complete the work in 1/4 hours
Together they can complete the work in 5/8 hours

So 1/12+1/x+1/4=5/8
1/x=5/8-1/3
1/x=7/24
X=24/7 which is 3 3/7

Sent from my SM-G925I using  GMAT Club Forum mobile app

JeffTargetTestPrep · Answer

The rate of machine X is 1/12, the rate of machine Y is 1/y, and the rate of machine Z is 1/4.

Since the 3 machines can complete the job in 1 3/5 = 8/5 hours, we can create the following equation for the combined rate of the 3 machines:

1/12 + 1/y + 1/4 = 1/(8/5)

1/12 + 1/y + 1/4 = 5/8

Multiplying by 24y, we have:

2y + 24 + 6y = 15y

24 = 7y

24/7 = 3 3/7 = y

Answer: C

jantwarog · Answer

For me the easiest way to solve it would be:

X does 1/12 of the work in 1 hour
Y does 1/y of the work in 1 hour
Z does 1/4 of the work in 1 hour

When they work together they can do the work in 1 3/5 hours -> 8/5 hours

Hence in 1 hour only 1/(8/5) of the total work is done, what means that (1/12+1/y+1/4)=5/8

it follows that 4/12+1/y=5/8
so, (5/8)-(4/12)=1/y
finding the common denominator -> (15/24)-(8/24)=1/y
7/24=1/y
y=24/7
y= 3 and 3/7

mm229966 · Answer

1/Y= (1/X + 1/Y + 1/Z) - (1/X + 1/Z) = 5/8 - (1/12 + 1/4) = 5/8 - 1/3 = 7/24 Y's Rate.
Y's time = 24/7 = 3 3/7

bumpbot · Answer

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Three machines X, Y, and Z operate independently of one another.

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Three machines X, Y, and Z operate independently of one another.

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