Find the sum of multiples of $5$ lying between $84$ and $719$.

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.

Write the first three terms of the $APs$ when $a$ and $d$ are as given below:
$a = \frac{1}{2}, d = -\frac{1}{6}$

For the finite $A.P.$ $3, 5, 7, \ldots, 201$,find the $12^{th}$ term from the end.

Can any term of the $A.P.$ $3, 7, 11, \ldots$ be $184$?

The sum of the first $n$ even natural numbers is........