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Question

Anjani can do a certain piece of work in 24 days. Anjani and Khushbu can together do the same work in 12 days, and Anjani, Khushbu and Sushmita can do the same work together in 9 days. In how many days can Anjani and Sushmita do the same work together?

This question was previously asked in
RRB NTPC 2024 CBT 1 Question Paper (28-Aug-2025) (Shift 3)
The correct answer is
$\frac{72}{5}$

Work and Time Problem: Finding Combined Duration

This problem involves calculating the time taken for two individuals, Anjani and Sushmita, to complete a piece of work together, given information about their individual and combined work rates.

Calculating Individual Work Rates

  • Anjani's time to complete the work = 24 days.
  • Anjani's work rate = $\frac{1}{24}$ of the work per day.
  • Anjani and Khushbu's combined time = 12 days.
  • Anjani and Khushbu's combined work rate = $\frac{1}{12}$ of the work per day.
  • Anjani, Khushbu, and Sushmita's combined time = 9 days.
  • Anjani, Khushbu, and Sushmita's combined work rate = $\frac{1}{9}$ of the work per day.

Determining Khushbu's Work Rate

To find Khushbu's individual rate, subtract Anjani's rate from their combined rate:

Khushbu's rate = (Anjani + Khushbu's rate) - (Anjani's rate)

Khushbu's rate = $\frac{1}{12} - \frac{1}{24} = \frac{2}{24} - \frac{1}{24} = \frac{1}{24}$ of the work per day.

Determining Sushmita's Work Rate

To find Sushmita's individual rate, subtract the combined rate of Anjani and Khushbu from the combined rate of all three:

Sushmita's rate = (Anjani + Khushbu + Sushmita's rate) - (Anjani + Khushbu's rate)

Sushmita's rate = $\frac{1}{9} - \frac{1}{12}$

Using a common denominator of 36:

Sushmita's rate = $\frac{4}{36} - \frac{3}{36} = \frac{1}{36}$ of the work per day.

Calculating Combined Rate of Anjani and Sushmita

Now, find the combined work rate of Anjani and Sushmita by adding their individual rates:

Anjani + Sushmita's rate = Anjani's rate + Sushmita's rate

Anjani + Sushmita's rate = $\frac{1}{24} + \frac{1}{36}$

Using a common denominator of 72:

Anjani + Sushmita's rate = $\frac{3}{72} + \frac{2}{72} = \frac{5}{72}$ of the work per day.

Final Time Calculation

The time taken for Anjani and Sushmita to complete the work together is the inverse of their combined rate:

Time = $\frac{1}{\text{Anjani + Sushmita's rate}} = \frac{1}{\frac{5}{72}}$

Time = $\frac{72}{5}$ days.

Therefore, Anjani and Sushmita can do the same work together in $\frac{72}{5}$ days.

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Important Questions from Time and Work

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