Let c be the amount of time the butterfly spends in a cocoon and l be the amount of time it spends as a larva. We know that l = 3c and l + c = 120.
Substituting the first equation into the second equation, we get 3c + c = 120.
Combining like terms, we get 4c = 120.
Dividing both sides by 4, we get c = 30.
#### 30