Why does changing a subrtraction problem to an addition with the complement of 9 work

Step-by-step explanation:When we need to subtract a number from another number, in that case, we can take the complement of the first number to add it to the second number, the result will be the same. It is because when we take the complement of 9 of that number, it will represent the negative of that number. Hence, by adding the negative of a number we will get the same result as we get after subtraction. For example: Subtract 213 from 843 843 - 213 = 630 complement of 9 of 213= 999-213                                        =786 Now, add 786 and 843 786+843=1629 We got the result in 4 digits so by adding the left-most digit to the right-sided three-digit number of the result, we will get 629+1 = 630