Add greater then in formula

I currently have the following formula:

If([Return Needed] and [Installation] = "Self-Install",
  [Addon] + [New] + [Replace],
  [Addon] + [New] - [Removal]
)

What I would like to add in the "If" statement, another "AND", with [Numberfield] <30

Kind of like this:

If([Return Needed] and [Installation] = "Self-Install", and [Numberfield] <30,

I tried different ways, and can't seem to get it.

[Numberfield] is a numeric formula, that displays end results in a number.