Problem: Find the cost of one pen if the cost of 20 pens is 356.80 Rs.

Solution:
To find the cost of one pen, we need to divide the total cost of 20 pens by the number of pens (20).

Step 1: Identify the given information:
- Cost of 20 pens: 356.80 Rs.
- Number of pens: 20

Step 2: Set up the equation:
Let's denote the cost of one pen as 'x'.
According to the given information, 20 pens cost 356.80 Rs. Therefore, we can form the equation:
20x = 356.80

Step 3: Solve the equation:
To find the value of 'x', we need to isolate it on one side of the equation. We can do this by dividing both sides of the equation by 20:
20x/20 = 356.80/20
x = 17.84 Rs.

Step 4: Interpret the result:
The cost of one pen is 17.84 Rs.

Summary:
- The cost of one pen is 17.84 Rs.
- To find the cost of one pen, we divided the total cost of 20 pens by the number of pens (20).
- The solution was obtained by setting up and solving an equation.