Please solve all parts, i'll give positive rating. please be quick

Q3 Q3 25 Points Bob has 500 dollars in his bank account. Every day, independently, Bob earns 2 dollars with probability 1/ 10, earns 4 dollars with probability 1/10 and loses 1 dollar with probability 4/5. (a) Find the number N for which the expected amount of money Bob has after N days is 400. (b) Approximate the probability that Bob has more than 415 dollars after N days (using the N from (a)) Express your answer using @(m). (c) Show that the probability that Bob has at least 500 dollars in his account after 11 days goes to 0 as n > oo