A zero order reaction starts at 0.36 mol/L…

Step 1. After 20 min. $[R] = 0.36\ \text{mol/L} - 0.0090\ \text{mol L}^{-1}\,\text{min}^{-1} \times 20\ \text{min} = 0.18\ \text{mol/L}$.

Step 2. Half-life. $t_{1/2} = \dfrac{0.36\ \text{mol/L}}{2 \times 0.0090\ \text{mol L}^{-1}\,\text{min}^{-1}} = 20\ \text{min}$.

Step 3. All used up. $t = \dfrac{0.36\ \text{mol/L}}{0.0090\ \text{mol L}^{-1}\,\text{min}^{-1}} = 40\ \text{min}$.