Interest Rate

A principal `P` that grows at rate `r` has a future value equal to $$V = P \left( 1 + r \right)$$

Example

You have borrowed $100 with 5% interest. You owe the bank: In total, you have to pay back $$100 \left( 1 + 0.05 \right) = 100 + 5 = \$105$$