#### Answer

a) $\frac{1}{10}$
b) $\frac{1}{2}$

#### Work Step by Step

a) There is an equal chance for all digits from $0$ to $9$ to be chosen. This is ten different numbers. Zero is one case out of the total ten, so the answer is $\frac{1}{10}$.
b) To get the probability that the last digit is greater or equal to 5, we have to add up the probabilities of it being 5,6,7,8,or 9. This is: $\frac{1}{10} + \frac{1}{10}+ \frac{1}{10}+ \frac{1}{10}+ \frac{1}{10} = \frac{5}{10} = \frac{1}{2}$