Discover Mathematics P4: Term 1, Unit 2, Lessons 1–4
GoalAdd big numbers with regrouping, and subtract by counting up and by a number line.
New words
regroup · to trade 10 of one place for 1 of the next placecounting up · finding a difference by counting up from the smaller number to the bigger onedifference · the answer to a subtraction
Explanation
When we add in columns, we start at the ones. If the digits in a place add to 10 or more, we regroup: write the ones digit and carry 1 to the next place. For 4,687 + 2,758: 7 + 8 = 15, write 5 and carry 1 ten. Tens: 8 + 5 + 1 = 14, write 4 and carry 1. Hundreds: 6 + 7 + 1 = 14, write 4 and carry 1. Thousands: 4 + 2 + 1 = 7. The sum is 7,445. To subtract, we can also count up. To find 800 − 468, start at 468 and count up to a friendly number, then on to 800. 468 to 470 is 2, 470 to 500 is 30, and 500 to 800 is 300. Add the jumps: 2 + 30 + 300 = 332. So 800 − 468 = 332. Check by adding: 468 + 332 = 800.
Worked examples
Place
Th
H
T
O
Carry
1
1
1
4
6
8
7
+
2
7
5
8
Sum
7
4
4
5
Add 4,687 + 2,758 in columns. The carries are 1 ten, 1 hundred and 1 thousand. The sum is 7,445.
A bakery sells 12,860 loaves in March and 9,475 loaves in April. Add: 12,860 + 9,475. Ones: 0 + 5 = 5. Tens: 6 + 7 = 13, write 3 and carry 1. Hundreds: 8 + 4 + 1 = 13, write 3 and carry 1. Thousands: 2 + 9 + 1 = 12, write 2 and carry 1. Ten thousands: 1 + 1 = 2. The bakery sells 22,335 loaves.
Mariam pays 600 LE for a school bag that costs 275 LE. To find her change, count up to find 600 − 275. From 275 count up 25 to 300, then 300 more to 600. 25 + 300 = 325. So 600 − 275 = 325. Check: 275 + 325 = 600.
Class activity
Count-up shop. Pretend you buy something for 275 LE and pay with 600 LE. The shopkeeper counts up to give the change: from 275 to 300, then to 600. Act it out with play money or paper notes and say each jump aloud. Try other prices.
Practice
1
Add 3,589 + 4,637 in columns.
8,226.
2
Add 25,408 + 17,693.
43,101.
3
Count up to find 500 − 372.
372 to 400 is 28, 400 to 500 is 100: 28 + 100 = 128.
4
Count up to find 900 − 645 and check by adding.
645 to 700 is 55, 700 to 900 is 200: 55 + 200 = 255. Check: 645 + 255 = 900.
5
Aya added 1,586 + 2,747 and got 3,223. Find her mistake and the right sum.
