Every digit has a place
In 42, the 4 means four tens and the 2 means two ones: 4 × 10 + 2 = 42. In 304, the 0 tells us there are no tens; it must not make the 4 move into the tens place. Hundreds, tens and ones combine to make one whole number.
Choose a useful range
Begin within 100 to read tens and ones, including the boundary at 100. Continue within 1,000 to include hundreds and a possible thousand. Enter the complete number, not just one digit, and check that each part contributes the right value.