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Question

Working $5\text{ h}$ a day, A can complete a task in $8\text{ days}$ and working $6\text{ h}$ a day, B can complete the same task in $10\text{ days}$. Working $8\text{ h}$ a day, they can jointly complete the task in:

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
$3\text{ days}$

Work Calculation: A's Effort

A completes the task working 5 hours a day for 8 days. The total hours A works is:

Total Hours for A = $5 \text{ h/day} \times 8 \text{ days} = 40 \text{ hours}$

Work Calculation: B's Effort

B completes the same task working 6 hours a day for 10 days. The total hours B works is:

Total Hours for B = $6 \text{ h/day} \times 10 \text{ days} = 60 \text{ hours}$

Combined Work Rate Calculation

Let the total amount of work required to complete the task be $W$ units.

  • A's work rate is $\frac{W}{40}$ units per hour.
  • B's work rate is $\frac{W}{60}$ units per hour.

When working together, their combined work rate is the sum of their individual rates:

Combined Rate = A's Rate + B's Rate = $\frac{W}{40} + \frac{W}{60}$

To add these fractions, find a common denominator, which is 120:

Combined Rate = $\frac{3W}{120} + \frac{2W}{120} = \frac{5W}{120} = \frac{W}{24}$ units per hour.

Joint Task Completion Time

A and B now work together, 8 hours a day. Let $D$ be the number of days they take to complete the task jointly.

The total hours they work together is $8 \times D$ hours.

The total work done is their combined rate multiplied by the total hours worked:

Total Work = Combined Rate $\times$ Total Hours Worked

$W = \frac{W}{24} \times (8 \times D)$

Simplify the equation:

$W = \frac{8DW}{24}$

Divide both sides by $W$ (assuming $W \neq 0$):

$1 = \frac{8D}{24}$

$1 = \frac{D}{3}$

Solving for $D$:

$D = 3$

Therefore, they can jointly complete the task in 3 days.

Solution Summary

The task requires 3 days to be completed when A and B work together for 8 hours daily.

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Important Questions from Time and Work

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