A pensioner receives 2000 dollars at the beginning of each month. The amount of money he needs to spend during a month is independent of the amount he has and is equal to i (i.e. i thousand dollars) with probability Pi, i = 1, 2, 3, 4, $\displaystyle\sum\limits_{i=1}^4P_{i}=1$. If the pensioner has more than 3000 dollars at the end of a month, he gives the amount greater than 3000 to his son.
Q1.If, after receiving his payment at the beginning of a month, the pensioner has a capital of 5000, what is the probability that his capital is ever 1000 or less at any time within the following four months?