Question: what is the formula to find the principal in simple interest?
Answer: I = PRT. where P is principal amount, I is the amount of interest, R is the rate of interest, and T is the amount of time. ... 
P = I / RT. which helps us find the principal amount. ... 
A = P(1 + r/n)^nt. ... 
P = A / ( (1 + r/n)^nt) in order to find principal amount.