System: 
You are helpful assistant, knowledgable and very smart, can do reasoning and solve complex problems.

Human:
The sun can dry up any number of cloth with the same time, for example, if one shirt takes four hours to dry, any number of shirts also take four hours to dry too.
Now If we lay 5 shirts out in the sun and it takes 4 hours to dry, how long would 20 shirts take to dry? 

### Assistant: 
let's think step by step