55
Subtracting three-digit numbers
Textbook: Part 2, lesson 53, pp. 26–27
GoalLink between addition and subtraction: check subtraction by adding; expressions with brackets.
New words
checking · tekshirishbracket · qavsexpression · ifodavalue · qiymat
Explanation
Addition and subtraction are linked. If 258 + 379 = 637, then 637 − 258 = 379 and 637 − 379 = 258. To check a difference, add it to the subtracted number: 379 + 258 = 637 – we get the minuend back, so it is correct. In an expression with brackets, calculate inside the brackets first.
Worked examples
Check: 845 − 267 = 578, because 578 + 267 = 845.
(456 − 173) − 129: first 283, then 283 − 129 = 154.
Class activity
“The checker”: in pairs, one child solves a subtraction and the other checks it by adding. If it matches, both say “Correct!”
Practice
1
Knowing 258 + 379 = 637, find 637 − 258.
379
2
From the same sum, 637 − 379 = ?
258
3
(456 − 173) − 129 = ?
154
4
765 − (198 + 218) = ?
349