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Two machines A and B work at constant rates which are in the ratio of 1:2 respectively. If the two machines work together, they can complete 1/3rd of a job in 3 hours. If machine A starts working on the job and works alone for 18 hours, how many hours would machine B working alone take to complete the remaining work?
  • a)
    3.5
  • b)
    4
  • c)
    4.5
  • d)
    5
  • e)
    5.5
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
Two machines A and B work at constant rates which are in the ratio of ...
Given:
  • Let the rate of machine A be x units of work per hour
    • Therefore the rate of machine B = 2x units of ork per hour
  • Let the total amount of work to be done be W units
    • Time taken by machines A and B working together to complete
  • Machine A works alone for 18 hours
To Find: Time taken by machine B to complete the remaining work
Approach:
  1. We know that Work = Rate * Time
c. So, to find the time taken, we either need to find the amount of remaining work and the rate of machine B or the ratio of 
2. For finding the remaining work, we need to find the amount of work done by machine A in 18 hours
  • Remaining work = W – work done by machine A in 18 hours
  • Work done by machine A in 18 hours = Rate of machine A * 18
    • As, rate of machine A is in terms of x, work done by machine A in 18 hours can be expressed in terms of x
  • Remaining work = W – work done by machine A in 18 hours
    • This will be an expression in terms of W and x. As the rate of machine B is in terms of x, we need to express W also in 
3. We are given that machines A and B working together take 3 hours to  complete W/3  units of work.
  •  We will use the above relation to express W in terms of x
Working out:
  1. Machines A and B working together take 3 hours to complete W/3 units of work
 
2. Amount of work done by machine A working alone for 18 hours = x * 18 = 18x
3. Remaining work = W – 18x = 27x – 18x = 9x
4. Amount of time taken by machine B to complete the remaining work = 
Answer : C
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Two machines A and B work at constant rates which are in the ratio of 1:2 respectively. If the two machines work together, they can complete 1/3rd of a job in 3 hours. If machine A starts working on the job and works alone for 18 hours, how many hours would machine B working alone take to complete the remaining work?a)3.5b)4c)4.5d)5e)5.5Correct answer is option 'C'. Can you explain this answer?
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