#### Explanation:

You need to use the correct formula to calculate simple interest.

$S I = \frac{P \times R \times T}{100}$

P= principal (starting amount)
R= rate of interest as a % - hence the '100'
T = time period, given in years
SI = simple interest.

$A = a m o u n t = P + S I$

Now just substitute in the given values and calculate.

$S I = \frac{255 \times 5 \times 1}{100}$

SI =\$12.75