Each pan of a balance holds a number or a small sum, and one number is missing. The student picks the number that makes the pans level; the beam tips as soon as a number is picked, so 'too big' and 'too small' can be seen.
It teaches = as 'the same as', not 'the answer comes next'. The missing number can be anywhere, even the first number of a subtraction: ? − 4 = 3 + 5.
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