Follow the healthy math practices that your child has been learning. For example, it is very important now that the digits in the problems be written in readable form, with ample space between problems that the columns can be kept straight. Crooked columns of numbers often lead to a tens-place digit, for example, being subtracted/added from/to a 100s-place digit. If your child keeps track with little numbers on top of the problem to show his/her regrouping efforts, there must be room for them to be legible. To work hard and not be able to read what has been accomplished, forcing one to start over, is very frustrating to children, and unnecessary.