Stock A and stock B.
Every day the price of stock A will either increase by 1 dollar with probability 0.6 or decrease by one dollar with probability 0.4. Similarly stock B will either increase by 2 dollars with probability 0.55 or decrease by two dollars with probability 0.45.

1. If, at the beginning, the prive of stock A was 100 dollars and that of stock B was 120 dollars, what is the (approximate) price distribution of stock A and B after 100 days?

Mu-stockA=.2 multiply by n=100 so Mu-stockA=20
Mu-stockB=20 as well

using this equation i found Stand.Dev.
Var^2=SUM( (Xi-mu)^2 * p(Xi) ) ....i then multiplied variance by n and took the entire square root to get...

2. Compute the probability that, after 100 days, the price of stock A will be less than than its initial price. Do the same for stock B.

I found probability P(price < 0)...i chose 0 b/c if it became negative that means stocks decrease ( i assume) and found the Z values
(0-20)/9.7979 = z-value of -2.04 so proba. =.0207
(0-20)/19.8997= z-value of -1.005 so probab. = .1587

3. At the beginning you have 20000 dollars and you buy 140 stock A and 50 stock B. What is the probability that after 100 days you have lost money? Doubled your initial capital?

can you hint me on how to start this one?