I am working on interest questions and there are a few I cannot figure out.

1) Calculate the present value of each investment, given the final value of the investment.

b) $10 775.64 after 50 years at 8% compounded monthly.

2) John loans $6000 to his sister, Sally. Sally agrees to pay him back 7 years later at a rate of 8% compounded weekly. How much interest does Sally have to pay.

3) An investment grows from $600 to $1200 in 9 years. If interest was compounded monthly, what was the annual rate?

I know I have to use the formula:

PV= A/(1+i)^n

but how?