It means you need to carry out the calculation in your head - without using a calculator or writing anything down.
calculating mentally, usually arithmetic
principal(in terms of math)- the amount you borrow or deposit
HL in math mean hypotenuse leg
ask your math teacher
It
In other words, do the math in your head, not on paper.
sum means the answer of a math problem
A number? ^^
neither
the average
Dekagrams.
average