What is the Double double and double strategy?

if you want to double something straight away you can add the numbers 2 times and it would be easyer if you add them in coulems. This is an exsample : double 397

you first double the numbers by adding them together (the numbers both have to be the same)

397 + 7+7= 14 put a 7 in the coulem. put a 1 in the 9 coulem
397
1 1
_____ 9+9= 18 pun an 8 in the coulem and a 1 in the hundrets coulem
add them up (theas ones are easy)