Second grade adds the third place. Every number is between 100 and 999, and each question asks for a digit's place, its value, or the whole number moved between standard form (352) and expanded form (300 + 50 + 2).
Zeros are where three-digit place value goes wrong - 305 is not thirty-five - so they turn up often. When these are secure, the four-digit Place Value page carries on into thousands.
Because that is where the mistakes are. A child who writes 305 as 3 + 5 has not yet seen that the zero is holding the tens place open.
Second grade, where three-digit place value is taught, with review at the start of third.
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