Q88. If $S = \{ a \in \mathbf { R } : | 2 a - 1 | = 3 [ a ] + 2 \{ a \} \}$, where $[ t ]$ denotes the greatest integer less than or equal to $t$ and $\{ t \}$ represents the fractional part of $t$, then $72 \sum _ { a \in S } a$ is equal to $\_\_\_\_$\\