Calculations that a person does in his head without the use of external aids is called mental math. It is the most common form of calculation in daily life, as it can quickly perform simple calculations and estimations.
Mental math uses facts about numbers that have been committed to memory. Such facts may include multiplication tables and relationships between numbers. For example, a student solving 86 x 5 might find it quicker to calculate the solution to (86 ÷ 2) x 10 than to write out the algorithmic process. Mental math is practiced in various ways, including finding different ways to calculate a given number.