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]]