Arithmetic Mean Return: investment portfolio, investment growth, stock returns

You solution for Problem #3 looks right to me, but for #2 is not. In #2 your computation of individual stock returns is incorrect.
For example if the price of stock D doubles every year what is its return after 3 years?
Thanks for taking the time to look at my work. To answer your question, let's assume stock D is valued at $n.

1st year: $2n
2nd year: $4n
3rd year: $8n

If n = $100, it would have grown to $800 after three years. My return would be $700.
So if your annual returns are 100% you get 700%, not 300% (100%+100%+100%), right? Then why are you adding up your annual returns for stocks A,B,C ?