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