The equation of a stationary wave is y = 0.8 cos $\left(\frac{\mathrm{\pi x}}{20}\right)$sin 200 πt where x is in cm and t is in s. The separation between consecutive nodes will be 

Standard equation
$\mathrm{y}=\mathrm{A} \cos \frac{2\mathrm{\pi x}}{\mathrm{\lambda }} \sin \frac{2\mathrm{\pi vt}}{\mathrm{\lambda }}$
By comparing this equation with given equation.
$\frac{2\mathrm{\pi x}}{\mathrm{\lambda }}=\frac{\mathrm{\pi x}}{20} \Rightarrow \mathrm{\lambda }=40 \mathrm{cm}$
Distance between two nodes = λ/2 = 20 cm