Answer
a) 900
b) $\frac{900}{1001}$
Work Step by Step
a) since the leftmost digit cannot be 0, there are $9 \times 10 \times 10 = 900$ ways to form a 3 digit number.
b) Since we know there are 900 3-digit numbers, and there are 1001 numbers in this set, the probability is $ \frac{900}{1001}$.
