57
Adding and subtracting three-digit numbers
Textbook: Part 2, lesson 55, pp. 30–31
GoalAdding and subtracting three-digit numbers: expressions, equations, problems.
New words
expression · ifodaequation · tenglamaunknown · noma’lumtotal · jami
Explanation
First find what the problem asks: “total” – add; “left” or “how many more” – subtract. In an equation: unknown addend = sum − known addend; unknown minuend = difference + subtrahend; unknown subtrahend = minuend − difference. If x + 765 = 926, then x = 926 − 765 = 161.
Worked examples
A theatre has 300 seats and 176 tickets were sold: 300 − 176 = 124 empty seats.
If z − 136 = 485, then z = 485 + 136 = 621.
Class activity
“Which sign?”: read a problem aloud; children hold up a “+” or “−” card and say the expression.
Practice
1
An actor must learn 168, 172 and 224 lines. How many lines in total?
564
2
Solve: x + 765 = 926. x = ?
161
3
Solve: 900 − y = 368. y = ?
532
4
Solve: z − 136 = 485. z = ?
621