After you multiply, draw a large X
next to the problem.
Add the digits in the first factor. If the
sum has more than one digit, add those digits. Write the one-digit
sum in the top space of the
X.
Add the digits in the second factor.
If they do not add up to a one digit number, add those digits until you
get a one-digit sum. Write the one-digit sum in the bottom space
of the X.
Multiply the one-digit number in the top
of the x by the one-digit number in the bottom of the x. Add the digits
of the product until you get a one-digit number. Write that number on the
left side of the X.
Add the digits in the product until you
get a one-digit number. Write that digit in the right side of the X.
Compare the left and right one-digit number
in the X. If they are different, you have definitely made a mistake.
If they are the same, you probably have not made a mistake. (But
you are not 100% sure.)
