From a month of 31 days, 3 different dates are selected at random. If the probability that these dates are in an increasing A.P. is equal to a/b,…

From a month of $\displaystyle 31$ days, $\displaystyle 3$ different dates are selected at random. If the probability that these dates are in an increasing A.P. is equal to $\displaystyle \frac{a}{b}$, where $\displaystyle a, b \in \mathbb{N}$ and $\displaystyle \operatorname{gcd}(a, b)=1$, then $\displaystyle a+b$ is equal to $\displaystyle \_\_\_\_$