Two pipes $P_1$ and $P_2$ fill a tank. $T_1 = \{2, 4\}$ hours and $T_2 = \{3, 6\}$ hours. If $t_1 \in T_1$ and $t_2 \in T_2$ are chosen, what is the set of possible times $t = \frac{t_1 t_2}{t_1 + t_2}$?