How to solve for P in a compound interest formula?

Calculation. Formula.

P (1 + r/n)^(nt)

Calculate accrued amount. Principal + Interest. A = P(1 + r)t

Calculate principal amount. Solve for P in terms of A. P = A / (1 + r)t

Calculate principal amount. Solve for P in terms of I. ...

Calculate rate of interest. As a decimal. ...

Calculate rate of interest. As a percent. ...

Calculate time. Solve for t.

area of part 1:

a1=base*height/2=(8+2)*(8-2-2-1)/2=10*3/2=15 ft^2

area of part 2:

a2=length*width=(8+2)*1=10 ft2

area of part 3:

a3=length*width=8*2=16 ft2

total shaded area : a1+a2+a3=41 ft2

total area of the reactangle : a=16*8=128 ft2

total area of the nonshaded region : 128-41=87 ft2