$3+6+9+12+\ldots+300 = \ldots$

The sum of three numbers in $A.P.$ is $48$ and the sum of their squares is $800$. Find those numbers.

The $n^{th}$ term of an $A.P.$ is denoted by:

Find the number of terms in the finite $A.P.$ $6, 5.5, 5, \ldots, -12$.

$A$ person has to clear a debt of $Rs. 3250$ by monthly instalments. In the first month he has to pay $Rs. 20$ and then onwards every instalment is $Rs. 15$ more than the previous instalment. How long will it take to clear the debt?

Find the sum of the first $20$ terms of the $A.P.$ $5, 13, 21, \ldots$ and find the number of terms of this $A.P.$ whose sum is $6440$.