Choose to subtract 3-digit numbers using formal written methods when appropriate

When teaching students to subtract 3-digit numbers, it is essential to choose the most efficient method, which can be either a formal written method or a mental strategy. The formal written method is typically used for complex calculations that may be too challenging to compute mentally. For example, subtracting numbers with different units, such as 576 - 323, is often more straightforward using the formal written method. In contrast, mental methods, which include calculation 'in your head' or with minimal 'jottings', are suitable for simpler subtractions where one can readily apply strategies like counting on or rounding.

Efficiency in subtraction also depends on the specific numbers involved. For instance, when subtracting a number with an 8 in the ones column, it might be suggested that a written method is more efficient, as it can help avoid errors in borrowing or regrouping. However, this is not a hard and fast rule, and some may argue that with practice, mental methods can be just as efficient. To determine the most appropriate method, one might consider the proximity of the numbers (e.g., 562 - 558) or whether the numbers lend themselves to easy mental adjustments (e.g., 546 - 399, where rounding can be applied). Ultimately, the choice between mental and written methods will depend on the individual's comfort level with the numbers and the complexity of the calculation.