What is the Arithmetic mean of the first $n$ natural numbers?

Two numbers differ by $5$. If their product is $336$,then the sum of the two numbers is

$2438.79 - 1233.99 + 399.99 = ? + 989.99$

The sum of squares of two numbers is $80$ and the square of their difference is $36$. The product of the two numbers is

There is a number consisting of two digits. The digit in the units place is twice that in the tens place,and if $2$ is subtracted from the sum of the digits,the difference is equal to $1/6^{\text{th}}$ of the number. The number is:

$(0.88 \times 880 \div 8) \times 6 = ?$