Addition and subtraction can be used using two stratigies: counting on/back and rounding and compensating.
Counting on is adding on using fingers, when the number is under ten. Counting back is subtraction using fingers, when the number is under ten.
Rounding and compensating is rounding the added or subtracted number to the nearest tidy number, then compensating for the rounding. Compensating is done by splitting the new number into the old number and the left over.
Counting on/back and rounding and compensating are easy stratigies are easy to use for addition and subtraction, when the numbers get too big to be solved with basic facts.
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