# Cell phones

Alignments to Content Standards: F-IF.A.2

Let $f(t)$ be the number of people, in millions, who own cell phones $t$ years after 1990. Explain the meaning of the following statements.

1. $f(10)=100.3$
2. $f(a) = 20$
3. $f(20) = b$
4. $n=f(t)$

## IM Commentary

This simple task assesses whether students can interpret function notation. The four parts of the task provide a logical progression of exercises for advancing understanding of function notation and how to interpret it in terms of a given context.

## Solution

1. The number of people who own cell phones in the year 2000 is $100,\!300,\!000$.
2. There are $20,\!000,\!000$ people who own cell phones $a$ years after 1990.
3. There will be $b$ million people who own cell phones in the year 2010.
4. The number $n$ is the number of people (in millions) who own cell phones $t$ years after 1990.