Terms on demand

You will write the formula for the $n$th term, a function that finds which term a number is, and use them to count.

The idea. $a_n = a + (n - 1)d$. To find which term a number $v$ is, $n = \dfrac{v - a}{d} + 1$, and it must be a whole number of at least $1$.

Your tasks.

1. In nth, return the $n$th term.
2. In which_term, return the term number of v as a whole number, or None when v is not a term. Assume d is not $0$.
3. The two-digit multiples of $7$ are $14, 21, \dots, 98$. Use which_term to set count to how many there are.