For ${\mathrm{CH}}_4+{\mathrm{Cl}}_2\stackrel{\text{ hν }}{⟶}$ product, rate of reaction at t = 0 is given by rate $=\frac{-\mathrm{d}}{\mathrm{dt}}\left[{\mathrm{CH}}_4\right]=2\times {10}^{-4}$ M-min^–1, then rate of the reaction at t = 20 min is (in M-min^-1)

Photo chemicl reactions are zero order reactions 

rate of reaction is same with respect to reactants and products 

for zero order reaction rate depends on external parameters

$\mathrm{Rate}=-\frac{\mathrm{d}\left[{\mathrm{CH}}_4\right]}{\mathrm{dt}}=-\frac{\mathrm{d}\left[{\mathrm{Cl}}_2\right]}{\mathrm{dt}}=+\frac{\mathrm{d}\left[\mathrm{product}\right]}{\mathrm{dt}}=2\times {10}^{-4}$mole/lit/min