Use rounding to subtract near multiples of 10

Using rounding to subtract near multiples of 10 is an effective mathematical strategy, particularly helpful when dealing with subtraction problems. This technique simplifies calculations by adjusting numbers to the closest tens, making mental arithmetic more manageable. For instance, when subtracting 299 from various numbers, one can observe that the process is akin to subtracting 300 and then adjusting the result by adding 1 back. This method is showcased in a series of calculations where numbers like 672, 419, and 672 are each reduced by 299, resulting in 373, 120, and 373 respectively, after the adjustment. Similarly, when subtracting numbers like 230 from 636, 752, 474, and 391, rounding to the nearest tens can streamline the process, and subsequent addition of 229 or 228 to these numbers further practices the concept.

Colin's approach to subtraction by rounding provides a clear example: he calculates 576 - 249 by first rounding 249 to 250, subtracting to get 326, and then compensating for the rounding by subtracting 1, resulting in a final answer of 325. This method is contrasted with an incorrect approach that overcompensates by subtracting too much. The material encourages learners to engage with the concept by filling in missing numbers in various subtraction problems, ensuring they understand the correct application of rounding. Problems such as 466 - 239 and 424 - 119 are solved using this method, yielding results of 227 and 305 respectively. The resource prompts students to create their own calculations where rounding to the nearest multiple of 10 is beneficial, reinforcing their understanding and application of the strategy.