Chinedu sat down to do his homework, which included $30$ math problems. He solved $2$ problems each minute.
Let $f(n)$ be the number of problems left for Chinedu to solve at the *beginning* of the $n^\text{th}$ minute.
**$f$ is a sequence. What kind of sequence is it?**
[[☃ radio 1]]
**Write an explicit formula for the sequence.**
$f(n)=$ [[☃ expression 1]]