$a$ is chosen from the set $\{1,2,3,4\}$, and $b$ is chosen from the set $\{5,6,7,8,9\}$. What is the probability that the product $ab$ is a prime number?
Question:
$a$ is chosen from the set $\{1,2,3,4\}$, and $b$ is chosen from the set $\{5,6,7,8,9\}$. What is the probability that the product $ab$ is a prime number?
Answer:
Let's think step by step.

user
\frac1{10}
$a$ is chosen from the set $\{1,2,3,4\}$, and $b$ is chosen from the set $\{5,6,7,8,9\}$. What is the probability that the product $ab$ is a prime number?
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