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Question

10 men and 15 boys together can complete a work in 6 days. It takes 100 days for one man alone to complete the same work. How many days will a boy alone take to complete the same work?

This question was previously asked in
RRB ALP 2025 CBT 2 Mechanic Motor Vehicle Question Paper (28-Jul-2026) (Shift 1)
The correct answer is
225

Work and Time Calculation

This problem involves calculating the time taken by a boy to complete a work, given combined work rates and individual work rates.

Man's Work Rate

We are given that one man alone can complete the work in 100 days.

The work rate of one man (\(M\)) is the reciprocal of the time taken:

\( M = \frac{1 \text{ work}}{100 \text{ days}} = \frac{1}{100} \text{ work/day} \)

Combined Work Rate

10 men and 15 boys together complete the work in 6 days.

This means their combined work rate is:

\( (10 \times M) + (15 \times B) = \frac{1 \text{ work}}{6 \text{ days}} = \frac{1}{6} \text{ work/day} \)

Where \(B\) is the work rate of one boy.

Calculating Boy's Work Rate

Substitute the known work rate of a man (\(M = \frac{1}{100}\)) into the combined work rate equation:

\( \left(10 \times \frac{1}{100}\right) + (15 \times B) = \frac{1}{6} \)

Simplify the term for 10 men:

\( \frac{10}{100} + 15B = \frac{1}{6} \)

\( \frac{1}{10} + 15B = \frac{1}{6} \)

Now, isolate \(15B\):

\( 15B = \frac{1}{6} - \frac{1}{10} \)

To subtract the fractions, find a common denominator (which is 30):

\( 15B = \frac{5}{30} - \frac{3}{30} \)

\( 15B = \frac{2}{30} = \frac{1}{15} \text{ work/day} \)

Now, find the work rate of a single boy (\(B\)):

\( B = \frac{1}{15 \times 15} = \frac{1}{225} \text{ work/day} \)

Time Taken by a Boy Alone

The time taken by one boy to complete the work is the reciprocal of his work rate:

\( \text{Time for 1 boy} = \frac{1}{B} = \frac{1}{1/225} \)

\( \text{Time for 1 boy} = 225 \text{ days} \)

Therefore, a boy alone will take 225 days to complete the same work.

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