This task is designed to get pupils thinking and understanding the associativity of subtraction. It is modeled both with number lines and directed counters. I believe that treating subtraction as the additive inverse, e.g. subtracting four is the same as adding on negative four, can really help pupils be successful here and can transfer well to future mathematics.
Question 1 uses 'varation' such that pupils attend to the operations and can begin to generalise, which is then tested in question 2, and built upon in question 3. Question 4 requires them to work backward and could help prime students to be more successful when expanding and simplifying two single brackets involving a subtraction.
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