A video explaining a Japanese multiplication trick has gone viral on Facebook with more than 400,000 shares.
The clip is actually taken from the popular YouTube series called DaveHax which shows how substituting numbers for lines can make complex mental arithmetic seem simple.
For example, 13 x 21. The first digit is one, which means you start by drawing a straight line diagonally.
The next digit is three, so you draw three lines parallel to the first one.
After the multiplication sign, the first number is two so draw two lines, like so.
The next number is one, so draw another line like so:
Then it needs to be put into three separate zones where the lines cross.
The answer is worked out by finding out how many times the lines cross in different zones, in this case 2-7-3. And 13x21 is... 273.
Watch the full video, which explains further methods, below: