Let * be a binary operation defined on $Q^{+}$(the set of positive rational numbers) by the rule $a * b=\frac{a b}{7}$ for all $a, b \in Q^{+}$. Then inverse of $2 * 7$ is:

7 $\frac{1}{7}$ $\frac{49}{2}$ $\frac{1}{14}$

$\frac{49}{2}$

The correct answer is Option (3) - $\frac{49}{2}$