Namaste to all,

We have heard of Chalisa, like Hanuman Chalisa, Gayatri Chalisa, Mahakali Chalisa, Maha Laxmi chalisa. I heard that all the chalisa have 40 verses, please correct me if I am wrong.

Why do have chalisa(40) and not pachasa(50) or sathsa(60) ? What is the purpose of Chalisa, I mean to say word chalisa ?