20
Adding and subtracting multi-digit numbers
Textbook: Part 1, lesson 20, pp. 61–63
GoalUse addition and subtraction of multi-digit numbers in equations and longer expressions.
New words
equation · tenglamaunknown · noma’lumexpression · ifodachecking · tekshirish
Explanation
To find an unknown minuend, add the subtrahend to the difference: x − a = b, x = b + a. To find an unknown subtrahend, subtract the difference from the minuend: a − y = b, y = a − b. In an expression we do brackets first, then multiplication and division, then addition and subtraction. We check by putting the answer back into the equation.
Worked examples
x − 28 450 = 41 300, so x = 41 300 + 28 450 = 69 750. Check: 69 750 − 28 450 = 41 300.
100 000 − y = 62 480, so y = 100 000 − 62 480 = 37 520.
Class activity
“Equation workshop”: pairs make up equations for each other (for example ○ − 17 500 = 30 000). The partner solves and checks.
Practice
1
x − 28 450 = 41 300. Find x.
69750
2
100 000 − y = 62 480. Find y.
37520
3
Multiply the sum of 38 and 47 by 6.
510
4
Calculate: 90 000 − (800 × 5 + 31 250).
54750
5
312 pupils visited a museum over 4 days. Day 1: 64; day 2: 15 more than day 1; day 3: 20 more than day 2. How many came on day 4?
70