In a Simple interest rate, where Money grows at fixed rate each year is given a linear equation, because rate of change is fixed ( throughout the year). A is the balance of the account after n( variable) years with a fixed rate( slope).
A = P + Pnr / here the n is the variable.
7800 = P( 1 +3(0.05)=
7800 = P + P( 0.15)
P = 7800 = 6783