If C=Cost, P=Principle (sales price), t=time and r = interest rate it's C=P*r*t +P.

When repaying a loan you have to pay the interest (Prt) plus the original principle.

If you borrowed $10000, the interest was 5% (.05) per year and you repaid after one year you'd pay:

C = 10,000*.05*1 + 10,000 = $500 + $10,000 = $15,000.
That is the original loan amount plus $500 in interest.

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