Are the following functions invertible $($an inverse exists$)?$

$f(t)$ is the number of customers in Macy's department store at $t$ minutes past noon on December $18,2000.$ Is this invertible?

$f(t)$ is the total accumulated rainfall in inches $t$ minutes into a sudden rainstorm in July, $2005.$

Is this invertible?

$3.f(w)$ is the cost of mailing a letter weighing $w$ grams.

Is this invertible?