Use THIS TEST and then read below.
Long before the age of computers, people used to calculate in their heads. There were many methods of mental math that allow astonishing math deeds.
As it is in our age, computers can crunch numbers much better than humans. Yet som dedicated sportsmen still train to excel in mental math. There are four disciplines of mental math: addition, multiplication, calendar operations and square root computation.
The training can improve your basic math skills and the way (speed and accuracy) your working memory processes numbers.
People who practice long enough often report synaesthetic experiences, like hearing the musing of numbers.
There are various ways to do mental math. For multiplication we suggest the Trachtenberg system, for addition you can use the mental Abacus. For square root there is a separate method (also here). Typically it is wise to check your calculation.
Do not read about the cool computation tricks without using them. In fact, test yourself before the learning first, and then slowly learn the math tricks and test yourself again. If you train well, you will multiply your score at least x10 within a month. Do not believe me? Start training! Test yourself using Rank your brain or equivalent.