Three containers $C_{1}$, $C_{2}$ and $C_{3}$ have water at different temperatures. The table below shows the final temperature $T$ when different amounts of water (given in liters) are taken from each container and mixed (assume no loss of heat during the process)
| $C_{1}$ | $C_{2}$ | $C_{3}$ | $T$ |
| $1l$ | $2l$ | -- | $60^{\circ}\mathrm{C}$ |
| -- | $1l$ | $2l$ | $30^{\circ}\mathrm{C}$ |
| $2l$ | -- | $1l$ | $60^{\circ}\mathrm{C}$ |
| $1l$ | $1l$ | $1l$ | $\theta$ |
The value of $\theta$ (in ${}^{\circ}\mathrm{C}$ to the nearest integer) is $\underline{\hspace{1cm}}$