Edit:

Damn, 2 minutes late

... ok, here's the solution:

m = amount of snow Muziek can shovel per minute

c = amount of snow Chick can shovel per minute

S = amount of snow

Muziek takes 40 minutes alone, hence we have

(Amount of snow) divided by (amount of snow per minute) = minutes, in other words

S/m = 40 minutes <-> m = S/40

When they both shovel, they take 30 minutes:

S/(c+m) = 30 minutes

Since we know from above that m = S/40, we can exchange m in the last equation and get

S / (c + S/40) = 30 minutes

From this follows:

c = S / 120

When Chick has to shovel alone (again, in purely hypothetical way), we get

S/c = S / (S/120) = 120 minutes = 2 hours.