A school teacher distributed $4$ pens (of different colours) per student. Find the number of pens with the teacher, if the number of students is $n$ and if she is left with zero pens.

 Look at the following matchstick pattern of letter $B$, the $B\text{'}s$ are not separated. Two neighbouring $B\text{'}s$ have two common matchsticks. Observe the pattern carefully and frame a rule for the same.

If $16$ candies were packed in each packet and the total number of candies is $x$, then find the number of packets used for packing.

After sharing $x$ pencils with your friend, you are still left with $7$ pencils. Express the total number of pencils before sharing.

If the cost of a table and a chair is $x$ and $y$ respectively, then find the total cost of $4$ chairs and a table.

Sanjay scored double the runs in test match $1$ than in test match $2$. If he scored one-third of the sum of the runs scored in test match $1$ and test match $2$, in test match $3$, then find his total runs in test match $3$.

Find a rule for finding the perimeter of an equilateral triangle. 

Find a rule for distributive property over subtraction and verify the same.

Find a general rule to find the perimeter of a rectangle whose length and breadth are $l$ and $b$, respectively.