What would be the percentage gain if a person bought books at a rate of 5 books for 18 rupees and sold them at a rate of 3 books for 12 rupees?

Sure, let's break it down:

Step 1: First, we need to find out the cost price (CP) and selling price (SP) of a single book.

The cost price of 5 books is 18 rupees, so the cost price of 1 book would be 18/5 = 3.6 rupees.

The selling price of 3 books is 12 rupees, so the selling price of 1 book would be 12/3 = 4 rupees.

Step 2: Now, we can calculate the gain. The gain is the selling price minus the cost price, so the gain would be 4 - 3.6 = 0.4 rupees.

Step 3: To find the percentage gain, we use the formula:

Percentage Gain = (Gain / Cost price) * 100

Substituting the values we have:

Percentage Gain = (0.4 / 3.6) * 100 = 11.11%

So, the percentage gain if a person bought books at a rate of 5 books for 18 rupees and sold them at a rate of 3 books for 12 rupees would be approximately 11.11%.