For a first order reaction $A \rightarrow B$

$t/\text{min}$	$[A]/M$
$0$	$0.6500$
$x$	$0.0650$
$20$	$0.00065$

$x=$ ______ min. (Nearest integer)

For a first order reaction, the rate constant $k$ is given by:

$k = \dfrac{1}{t} \ln \dfrac{[A]_0}{[A]_t}$

Using the data at $t = 20$ min:

$k = \dfrac{1}{20} \ln \dfrac{0.6500}{0.00065} = \dfrac{1}{20} \ln 1000 = \dfrac{3}{20} \ln 10$

Using the data at $t = x$ min:

$k = \dfrac{1}{x} \ln \dfrac{0.6500}{0.0650} = \dfrac{1}{x} \ln 10$

Equating the two expressions for $k$:

$\dfrac{3}{20} \ln 10 = \dfrac{1}{x} \ln 10$

$\dfrac{3}{20} = \dfrac{1}{x}$

$x = \dfrac{20}{3} = 6.66...$

Rounding to the nearest integer, we get $x = 7$.

Answer: $7$