Explanation:

Let’s break this down step by step:

- A’s efficiency: 50 units in 10 hours ? 5 units/hour.

- B’s efficiency: 60 units in 12 hours ? 5 units/hour.

- Minimum efficiency (A and B): That’s 5 units/hour.

- Average of minimum efficiency: Since both are the same, average is also 5 units/hour.

- C’s efficiency: 20% more than 5 = 5 + 1 = 6 units/hour.

Now comes the time management:

- C works for 15 hours (from table: "Maximum time required by each to complete the work," C = 15 hours).

- Each work + rest cycle for C is: 1 hour work + 10 minutes rest = 1 hour 10 min = 1.1667 hours/cycle.

- How many complete 1-hour work cycles fit in 15 hours?

- Total time spent (per cycle): 70 minutes

- Number of complete cycles: 15 hours × 60 minutes/hour = 900 minutes ÷ 70 minutes/cycle ˜ 12.857 cycles.

- That’s 12 full cycles (12 hours work, 12 × 10 min = 120 min = 2 hrs rest used up), and C will have 1 hour left (as the next cycle isn’t fully possible).

- So, C actually works for 12 + 1 = 13 hours (last setup, work for the final hour but doesn’t get to rest after).

- Total work: 13 hours × 6 units/hour = 78 units.

Options review:

1. 90 – too high

2. 64 – too low

3. 82 – close but not matching math

4. 78 – matches our result

5. None – not needed

Correct Answer: Option 4, 78 units

That’s actually the answer. Here’s what’s going on at every step, and the math fits precisely. 78 is the work C does, factoring in the breaks and the efficiency boost. None of the quirks of the problem throw this off.