Are the following functions invertible?

1. $f(d)$ is the number of orange woolen hats sold at a department store on the $d$th day after September 1, 2003.
Is this invertible? ? yes no

2. $f(n)$ is the number of students in your calculus class whose birthday is on the $n$th day of the year.
Is this invertible? ? yes no

3. $f(w)$ is the cost of mailing a letter weighing $w$ grams.
Is this invertible? ? yes no