How long will it take the sun to burn off the fog if it can supply Q =40 J s-1 to the 1 m3 of air with fog?

q = 113 joules (energy required to evaporate all liquid water in 1m3 of fog)

* refer to your other post on how we calculated this energy.

if it is given that sun can supply Q =40 J s-1, then it needs 113/40 = 2.825 sec to burn off 1m3 of air with fog.

==> Answer = 2.825 sec for the sun to burn off 1m3 of air with fog.