For the following $APs,$ $3, 1, -1, -3, \ldots$ write the first term and the common difference.

How many two-digit numbers are divisible by $3$?

In an $AP$,given $l = 28$,$S = 144$,and there are a total of $9$ terms. Find $a$.

$A$ manufacturer of $TV$ sets produced $600$ sets in the third year and $700$ sets in the seventh year. Assuming that the production increases uniformly by a fixed number every year,find the production in the $1^{st}$ year.

Which term of the $AP: 21, 18, 15, \ldots$ is $-81$? Also,is any term $0$? Give reason for your answer.

In which of the following situations,does the list of numbers involved make an arithmetic progression,and why? The amount of air present in a cylinder when a vacuum pump removes $\frac{1}{4}$ of the air remaining in the cylinder at a time.