$3.$ In a warehouse, boxes are stored such that $30$ boxes on the bottom row and $21$ on the top row. There are $10$ rows in all, with each row having one more box than the one above it$.$

a$)$ How many boxes have been stored?

b$)$ Briefly explain the technique you used to solve $($a$).$

$4.$ A businessman has done an investment of Rs$\mathrm{.}200,000\mathrm{.}00$ on a new business expecting a $5\%$ interest compounded yearly.

a$)$ Find the total amount of money that the businessman would earn in $6$ years.

b$)$ Briefly explain the technique you used to solve $($a$).$