Simple Interest is always calculated as a percentage of the original amount.
Simple Interest is calculated over a fixed period, such as a month, or a year. If multiple periods (eg 7 years) are involved, then the total interest is a multiple of the interest amount for one period (7 x interest for 1 year).
Jacob has £150 in an account earning 2.5% simple interest per annum (each year). How much interest does he earn over 4 years?
In one year he earns `150 xx frac(2.5)(100)` = £3.75
Over four years, 4 x 3.75 = £15.00
Robyn earned £24 in simple interest from her savings over 6 years. If the interest rate was 2.5%, what was her original investment?
Amount earned in interest each year is 24 ÷ 6 = £4.00
Interest earned in one year: `4 = text(amount) xx frac(2.5)(100)`
4 x 100 ÷ 2.5 = amount
See also Percentage Increase and Decrease