Predicting the graph of the given reaction 
I have no idea on how to answer this question. Please help.

The concentration must be exponential. Suppose $C(t)$ is the concentration at time $t$ (min). $C(t)$ is of the form
$$
C(t) = A2^{-kt}
$$
where $A$ and $k$ are constants you need to find. Using known data points, you can find that $A = 0.4$ and $k = \frac 1{120}$.
The slope of $C(t)$ at $t = 0$ is $C'(0)$, so compute $C'(t)$ first:
\begin{align}
C'(t) & = -kA 2^{-kt} \log 2 \\
C'(0) & = -\frac 1{120} \cdot 0.4 \cdot \log 2 \approx -0.00231.
\end{align}
Therefore, the tangent line at $0$ would intersect the $x$-axis at
\begin{align}
-\frac{C(0)}{C'(0)} & \approx \frac{0.4}{0.00231} \approx 173.12
\end{align}
