Explanation:

- To solve this, use the formula for simple interest: A = P(1 + rt), where A is the amount, P is the principal, r is the rate, and t is the time in years.

- The amount A becomes 5 times the principal P after 20 years.

- This means A = 5P.

- Plugging into the formula: 5P = P(1 + r*20).

- Dividing both sides by P, we simplify it to: 5 = 1 + 20r.

- Solving for r gives: 4 = 20r, therefore r = 4/20 = 0.2 or 20%.

- Thus, the correct rate of simple interest per annum is 20%.

Option 3: 20% is the correct answer.