Find the $16^{th}$ term and the $n^{th}$ term of the $A.P.$ $9, 12, 15, \ldots$

In each of the following,$a$ and $d$ for an $A.P.$ are given. Find the $A.P.$ in each case. $a = 21, d = -7$.

In which of the following situations,do the lists of numbers involved form an $AP$? Give reasons for your answers.
$(i)$ The fee charged from a student every month by a school for the whole session,when the monthly fee is $Rs. 400$.
$(ii)$ The fee charged every month by a school from Classes $I$ to $XII$,when the monthly fee for Class $I$ is $Rs. 250$,and it increases by $Rs. 50$ for the next higher class.
$(iii)$ The amount of money in the account of Varun at the end of every year when $Rs. 1000$ is deposited at simple interest of $10\%$ per annum.
$(iv)$ The number of bacteria in a certain food item after each second,when they double in every second.

The $n^{th}$ term of the $A.P.$ $2, 7, 12, 17, \ldots$ is ..........

How many terms of the $A.P.$ $8, 15, 22, \ldots$ add up to $1490$?

For an $A.P.$,the first term is $5$ and the common difference is $3$. Then,the $15^{th}$ term of the $A.P.$ is..........