3
Three-digit subtraction
Discover Mathematics P2: Term 2, Lessons 91β100
GoalSubtract three-digit numbers, trading a ten or a hundred when a place has too few.
New words
hundreds Β· groups of one hundred in a numbertrade Β· take one from the next place to the left and make ten in this placecheck Β· 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 β 127: the ones 2 β 7 cannot, so trade a ten. Now 4 tens and 12 ones. 12 β 7 = 5. Tens: 4 β 2 = 2. Hundreds: 3 β 1 = 2. So 352 β 127 = 225. Check: 225 + 127 = 352.
Worked examples
478 β 235 needs no trading. Ones: 8 β 5 = 3. Tens: 7 β 3 = 4. Hundreds: 4 β 2 = 2. So 478 β 235 = 243.
463 β 218: the ones 3 β 8 cannot, so trade a ten. Now 4 hundreds, 5 tens and 13 ones, as in the picture. 13 β 8 = 5, 5 β 1 = 4, 4 β 2 = 2. So 463 β 218 = 245.
530 β 245: trade a ten for the ones, then a hundred for the tens. 10 β 5 = 5, then 12 β 4 = 8, then 4 β 2 = 2. So 530 β 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 β 243.
586 β 243 = 343.
2
Find 372 β 145.
372 β 145 = 227.
3
Seifβs shop has 540 bottles of water. It sells 265. How many are left?
540 β 265 = 275 bottles.
4
Find 421 β 236, then check by adding.
421 β 236 = 185. Check: 185 + 236 = 421.
5
A book has 312 pages. Nour has read 148. How many pages are left?
312 β 148 = 164 pages.