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Question

Sita can clean a hotel in 20 days. Rita can clean the same hotel in 30 days. How many days will they take to clean the hotel together?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
12 days

Calculating Joint Work Time for Hotel Cleaning

This problem involves calculating the time it takes for two individuals, Sita and Rita, to complete a task (cleaning a hotel) when working together. We determine their individual rates of work and then combine these rates to find their joint rate.

Sita's Work Rate

Sita can clean the hotel in 20 days. Her work rate is the fraction of the hotel she cleans per day.

Sita's rate = $\frac{1}{\text{Days Sita takes}} = \frac{1}{20}$ hotel per day.

Rita's Work Rate

Rita can clean the same hotel in 30 days. Her work rate is:

Rita's rate = $\frac{1}{\text{Days Rita takes}} = \frac{1}{30}$ hotel per day.

Combined Work Rate

To find how fast they work together, we add their individual rates.

Combined rate = Sita's rate + Rita's rate

Combined rate = $\frac{1}{20} + \frac{1}{30}$

To add these fractions, we find a common denominator, which is 60.

Combined rate = $\frac{3}{60} + \frac{2}{60} = \frac{3+2}{60} = \frac{5}{60}$

Simplifying the combined rate:

Combined rate = $\frac{1}{12}$ hotel per day.

Time Taken Together

The time taken to complete the job together is the reciprocal of their combined rate.

Time together = $\frac{1}{\text{Combined rate}}$

Time together = $\frac{1}{\frac{1}{12}} = 12$ days.

Conclusion

Working together, Sita and Rita will take 12 days to clean the hotel.

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