An=an-1-3
I hope this helps.
Each room would be 1 hour because it say that two rooms get painted in two hours <span />
Use present value equation.
If present value of annuity equals 400,000, then you are indifferent.

where v = 1/(1+r) and A = annual payment = 800,000/20 = 40,000
PV = 400,000
After subbing in the numbers, the equation is:

Therefore at interest rate of about 7.75% you are indifferent.
The sum of two rational numbers will always be rational. A rational number is a number that can be expressed in the form a/b with b ≠ 0. So, if we can prove that the sum of two rational numbers has a denominator, we have proven this statement. Since any rational number can be written as a/b, we can write sum of two rational numbers as a/b + c/d. We can make this ad+bc/bd. Since we proved that the sum of two rational numbers has a denominator, we also prove that their sum is rational.