A proposed answer to an addition or subtraction problem within 20 can be verified by representing the quantities with objects, joining or separating the collections, and counting the result; the count must correspond to the claimed numeral. This builds one-to-one correspondence between quantities and numerals, supports noticing and correcting counting errors, and introduces checking through the inverse relationship between addition and subtraction without extending to abstract proofs or larger-number algorithms.
To check an addition answer, use objects such as counters, blocks, or buttons.
Suppose the problem is:
Make a group of 8 objects.
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Make a second group of 5 objects.
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Join the two groups.
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Count all the objects once, touching each object as you count:
The objects make a group of 13. This matches the claimed answer, so is correct.
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