Three numbers form an arithmetic progression with sum $21$ and product $315$ . What is the absolute value of the common difference?

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A geometric sequence has first term $2$ and common ratio $3$ . What is the sum of its first $5$ terms?
Two fair dice are rolled. What is the probability that the sum is $8$ ?
In how many ways can a committee of $3$ people be chosen from $5$ candidates?
A bag holds $4$ red and $6$ blue balls. Two balls are drawn without replacement. What is the probability that both are red?
For events $A$ and $B$ , $P (B) = 0.4$ and $P (A \cap B) = 0.1$ . What is $P (A ∣ B)$ ?
Let $a = (1, 2)$ and $b = (3, - 1)$ . What is $a \cdot b$ ?