5 + 8s = 15 + 3s
8s - 3s = 15 - 5
5s = 10
s = 10/5
s = 2 <=== they will charge the same at 2 sq ft
A=Pe^(rt)
P = 800g
t = 8 years
A = 450g
r = This is what we will try and find to start with
450=800e^(r*8)
After running the math through a calculator, we end with r = -0.07192
Now we just re-input this information into our equation: A=800e^(-0.07192*16)
A=800e^(1.15072)
Now we will re-write the equation using the negative exponent rule:
A = 800 1/e^1.15072
Combine right side:
A = 800/e^1.15072
Then do the math:
A = 253.12709836......
That will give us A = 253 (rounded to the whole number)
I hope this helps! :)
