Interest=ptr/100 where p=principal, t the number time units (usually years), r=%ge rate for the time unit (usually annual). r/100 converts the %ge into a fraction. Each time unit increases the interest by the same fraction of the original principal in simple interest. In this case 3/25 is r as a fraction, so r=100(3/25)=12%.
The interest increases by 12% of the principal for each time period.
After the time period t the interest is ptr/100=3pt/25, so the principal increases to p+3pt/25.
This expression factorises: p(1+3t/25). If t=25 years and the annual interest rate is 12% then the amount becomes p(1+3)=4p, that is, 4 times the original principal. When t=1 (settlement after one year), then p(1+0.12)=1.12p is the amount after a year. If p=$1000, for example, then it grows to $1120 after a year.
(Usually interest is compounded monthly or quarterly, and simple interest does not apply. But when t=1 and r is the annual rate, simple interest does apply.)