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This problem is about work done by a group of people, and we can solve it using the formula for work, which is:

\[
\text{Work} = \text{Number of men} \times \text{Hours per day} \times \text{Number of days}
\]

Let’s break down the information:

- 40 men working 8 hours per day for 18 days can dig the pond. 
- We are asked to find how many hours per day 36 men should work in 20 days to dig the same pond.

### Step 1: Calculate the total work done by the first group.
For 40 men, the work done is:

\[
\text{Work}_1 = 40 \, \text{men} \times 8 \, \text{hours/day} \times 18 \, \text{days}
= 5760 \, \text{man-hours}
\]

### Step 2: Calculate the required work for 36 men in 20 days.
Let \( x \) be the number of hours per day that 36 men must work to complete the same amount of work. The total work they do must equal 5760 man-hours. So:

\[
\text{Work}_2 = 36 \, \text{men} \times x \, \text{hours/day} \times 20 \, \text{days} = 5760 \, \text{man-hours}
\]

### Step 3: Solve for \( x \).
Now, set the two equations equal to each other:

\[
36 \times x \times 20 = 5760
\]

Simplifying:

\[
720x = 5760
\]

\[
x = \frac{5760}{720} = 8
\]

Thus, the number of hours per day that 36 men must work is **8 hours**.

### Answer: 
The correct option is **[D] 8 hours**.

Would you like further details, or do you have any questions? 

Here are 5 related questions to practice:
1. If 50 men worked 9 hours per day, how many days would they take to dig the same pond?
2. If 36 men can dig the pond in 15 days, how many hours per day should they work?
3. How long would 20 men take, working 6 hours a day, to complete half of the pond?
4. If the number of men is doubled, how does that affect the time taken to complete the work, assuming hours per day remain constant?
5. What is the total amount of work in man-hours required to dig the pond?

**Tip:** In work-related problems, the product of men, hours per day, and days is always constant for a fixed amount of work.

40 men working 8 hours per day can dig a pond in 18 days. By working how many hours per day can 36 men dig the same pond in 20 days?

Learn how to solve a work rate problem where 36 men need to dig a pond in 20 days. This solution involves calculating hours per day using proportional relationships and algebraic formulas. Perfect for students in Grades 8-10.

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