3
Three-digit subtraction
Discover Mathematics P2: Term 2, Lessons 91–100
GoalSubtract three-digit numbers, trading a ten or a hundred when a place has too few.
New words
hundreds · groups of one hundred in a numbertrade · take one from the next place to the left and make ten in this placecheck · use adding to see if a subtraction is right
Explanation
To subtract three-digit numbers, start with the ones, then the tens, then the hundreds. If a place has too few, trade from the place on its left: 1 hundred for 10 tens, or 1 ten for 10 ones. For 352 − 127: the ones 2 − 7 cannot, so trade a ten. Now 4 tens and 12 ones. 12 − 7 = 5. Tens: 4 − 2 = 2. Hundreds: 3 − 1 = 2. So 352 − 127 = 225. Check: 225 + 127 = 352.
Worked examples
478 − 235 needs no trading. Ones: 8 − 5 = 3. Tens: 7 − 3 = 4. Hundreds: 4 − 2 = 2. So 478 − 235 = 243.
463 − 218: the ones 3 − 8 cannot, so trade a ten. Now 4 hundreds, 5 tens and 13 ones, as in the picture. 13 − 8 = 5, 5 − 1 = 4, 4 − 2 = 2. So 463 − 218 = 245.
530 − 245: trade a ten for the ones, then a hundred for the tens. 10 − 5 = 5, then 12 − 4 = 8, then 4 − 2 = 2. So 530 − 245 = 285. Check: 285 + 245 = 530.
Class activity
Place-value mat. Draw three columns on paper: hundreds, tens, ones. Show 352 with 3 big paper squares, 5 paper strips and 2 small squares. Take away 127. You must trade one strip for ten small squares. Say the new number each time you trade.
Practice
1
Find 586 − 243.
586 − 243 = 343.
2
Find 372 − 145.
372 − 145 = 227.
3
Seif’s shop has 540 bottles of water. It sells 265. How many are left?
540 − 265 = 275 bottles.
4
Find 421 − 236, then check by adding.
421 − 236 = 185. Check: 185 + 236 = 421.
5
A book has 312 pages. Nour has read 148. How many pages are left?
312 − 148 = 164 pages.