Question : P, Q, and R can complete a work alone in 12,15, and 20 days respectively. They started the work together. P left the work 8 days before the work was completed and Q left the work 5 days after P had left. R completed the remaining work alone. How many days will be required to complete the whole work?
Option 1: $\frac{32}{3}$ days
Option 2: $10$ days
Option 3: $\frac{28}{3}$ days
Option 4: $\frac{25}{3}$ days
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Correct Answer: $\frac{28}{3}$ days
 
  Solution :
 
 Given that P can complete the work in 12 days, Q can complete the work in 15 days, and R can complete the work in 20 days.
 
  P left the work 8 days before it was completed, Q left the work 5 days after P had left, and R completed the remaining work alone.
  
  Total work = LCM$(12, 15, 20) = 60$
  
  Efficiency of P = $\frac{60}{12} = 5$
  
  Efficiency of Q = $\frac{60}{15} = 4$
  
  Efficiency of R = $\frac{60}{20} = 3$
  
  Let the total work be done in $t$ days.
  
  According to the question,
  
  ⇒ $5(t - 8) + 4(t - 3) + 3t = 60$
  
  ⇒ $5t - 40 + 4t - 12 + 3t = 60$
  
  ⇒ $12t = 112$
  
  ⇒ $t = \frac{112}{12} = \frac{28}{3}$ days
  
  Hence, the correct answer is $\frac{28}{3}$ days.
 
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