Answer:
Step-by-step explanation:
Two accounts start with a balance of $1000. One earns 8.05% interest compounded monthly; the other earns 8% compounded continuously. You want to know how long before the account balances differ by $100 and by $100,000.
The balance in the 8.05% account is given by ...
A = 1000(1 +0.0805/12)^(12t)
This can be written in terms of equivalent continuously compounded interest as ...
A = 1000e^(at) . . . . . . . where a=12·ln(1 +0.0805/12) ≈ 0.0802312
The balance in the 8% account is given by ...
A = 1000e^(0.08t)
The difference between the two account balances is ...
1000e^(at) -1000e^(.08t) = d
Dividing by the second term gives ...
e^((a-0.08)t) -1 = d/1000e^(-.08t)
And subtracting the term on the right gives a function of time that we can set to zero:
f(t) = 0
f(t) = e^((a-0.08)t) -1 -d/1000e^(-.08t)
This equation has no algebraic solution. The attached graph shows graphical solutions for d=100 and d=100,000.
$100 difference — 32.4 years
$100,000 difference — 104.0 years