Q1.
Evaluate 15 – 5, 100 – 10 and 74 – 34 from this perspective.
Answer
‘This perspective’ means reading a subtraction as finding the missing number to be added — how much must the smaller number grow to become the bigger one?
15 – 5 → 5 + ? = 15 → ? = 10
100 – 10 → 10 + ? = 100 → ? = 90
74 – 34 → 34 + ? = 74 → ? = 40
100 – 10 → 10 + ? = 100 → ? = 90
74 – 34 → 34 + ? = 74 → ? = 40
Why this way of looking helps: ‘take away’ makes no sense when you want (– 1) – (+ 3) — you cannot take 3 positives away from – 1 easily. But ‘what must be added to + 3 to reach – 1?’ always makes sense: the answer is the movement – 4. This is the meaning of subtraction used through the whole chapter.
Tip: In lift language, 74 – 34 = 40 says: to travel from Floor 34 to Floor 74 you must press + 40.