Principal: The money borrowed or lent out for a certain period is called the principal or the sum.
Interest: Extra money paid for using other's money is called interest.
Simple Interest (S.I): If the interest on a sum borrowed for certain period is reckoned uniformly, then it is called simple interest.
Let Principal = P, Rate = R% per annum and Time = T years. Then
(i) Simple Intereest (S.I.) = (P * R * T)/100
(ii) P = (100 * S.I)/(R * T)
R = (100 * S.I)/(P * T)
T = (100 * S.I)/(P * R)