Solve missing number problems for addition and subtraction within 20, with missing numbers in all positions.

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Celtic Counting (counting, proof)

by MathPickle

1.OA.D.8: Determine the unknown whole number in an addition or subtraction equation relating three whole numbers. *For example, determine the unknown number that makes the equation true in each of the equations 8 + _ = 11, 5 = _ - 3, 6 + 6 = _.*