For the reaction $\mathrm{A}\to$ products,$-\frac{d\left[\mathrm{A}\right]}{dt}=k$ and at different time interval, [A] values are :

Time	0	5 min	10 min	15 min
[A]	20 mol	18 mol	16 mol	14 mol

At 20 minute, rate will be :

The concentration varies by 2 moles per litre every five minutes.
As a result, the rate of reaction is constant.

$\text{ Rate }=\frac{\text{ Change in concentration }}{\text{ Time interval }}$

$\Rightarrow \frac{2}{5}=0.4\mathrm{mol}/\min$