Based on the information given the interest rate that you charge your friend is 12.5%.
Using this formula
Interest rate=Pay back amount- Loan amount/Loan amount
Where:
Pay back amount=$270
Loan amount=$240
Let plug in the formula
Interest rate=$270-$240/$240
Interest rate=$30/$240
Interest rate=0.125×100
Interest rate=12.5%
Inconclusion the interest rate that you charge your friend is 12.5%.
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