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Interest which is paid only on the principal and not on the additional amount generated by previous interest payments. A formula for computing simple interest is
where is the sum of principal and interest at time for a constant interest rate .
More things to try:
Weisstein, Eric W. "Simple Interest." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SimpleInterest.html
is the sum of principal and interest
at time 
for a constant interest rate 
.
