Which Term?

The rule turns a position into a term. Sometimes you know the term and want the position. In 4, 9, 14, ..., the rule is $a\left(n\right) = 4+\left(n-1\right)\cdot 5$. Which term is 79?

Set the rule equal to 79 and solve for n: $4+\left(n-1\right)\cdot 5 = 79$. Move the 4 across to get $\left(n-1\right)\cdot 5 = 75$, divide by 5 to get $n-1 = 15$, and add 1: $n = 16$. So 79 is the 16th term.

The answer has to be a whole number, 1 or more, because positions are 1st, 2nd, 3rd and so on. If n comes out as 15.4, or as 0 or below, the number is not in the sequence at all.