Use rounding to subtract near multiples of 10

The ArithmeKit 215 from Buzzard Publishing, part of the CanDoMaths series, offers a mathematical exercise that focuses on using rounding to simplify subtraction involving numbers close to multiples of 10. In this exercise, students are encouraged to match calculations and observe patterns in the results. For example, when they subtract 10 from a list of numbers like 64, 53, 75, 81, and 43, and then subtract 9 from the same numbers, they are meant to notice that the answers are just one more than when they subtracted 10, due to the subtraction of 9 being nearly the same as subtracting a multiple of 10. This principle is extended to subtracting 20 and then 19 from another set of numbers, reinforcing the concept that subtracting a near multiple of 10 can be made easier by rounding.

The kit further challenges students with problems that require them to apply this method of rounding and adjusting. For instance, they have to convince a character named Coco that 45 minus 9 is the same as 45 minus 10 plus 1, and not 45 minus 10 minus 1. This concept is tested with various numbers, encouraging students to create their own matching pairs problems and missing digit problems. Additionally, the exercise prompts students to think critically about when this method is most effective and explores the flexibility of problem-solving by asking if there's more than one way to solve a given problem using all the digits from 0 to 9 at least once. The kit not only helps students practice subtraction in a fun and engaging way but also enhances their understanding of rounding and its practical applications in arithmetic.