Randomly draw a card twice with replacement from $1$ to $10$, inclusive. What is the probability that the product of these two cards is a multiple of $7$?
Clearly, the number of total possibilities is $10\times 10=100$.
There are multiple ways to count the number of qualified cases:
List all the cases depending on whether each draw is $7$
Therefore, the total count is $1 + 9 + 9 = 19$ and the probability is $\boxed{\frac{19}{100}}$.
A little improvement over the previous method
We note that if the first card is $7$ then we do not need to care about the second card. Therefore, the total count can also be calculated as $$1\times 10 + 9\times 1 = 19$$
Count the opposite
If and only if neither card is $7$ will make their product not a multiple of $7$. There are $9\times 9=81$ such cases. Therefore, the qualified cases count is $100-81=19$.
The probability way
The event that the first card drawn is $7$ and the second card drawn is $7$ are indpendent events with probability of $\frac{1}{10}$. Therefore the probability that at least one of them occurs is $$\frac{1}{10} +\frac{1}{10} -\frac{1}{10}\times\frac{1}{10}=\frac{19}{1000}$$