Mental Math & Estimation
Solve mentally:
199 + 46
Answer: 245
Possible strategy: 200 + 46 = 246, then subtract 1.
426 + 351
777
786 − 243
543
402 − 176
226
A school has 2,475 students. Another school has 1,328 students.
How many students are there altogether?
3,803 students
Solve mentally:
3,998 + 527
Answer: 4,525
Possible strategy: 4,000 + 527 = 4,527, then subtract 2.
2,748 + 3,596
6,344
652 − 378
274
703 − 458
245
A library had 6,250 books. Students checked out 1,875 books.
How many books remain?
4,375 books
Estimate by rounding each number to the nearest thousand:
6,283 + 2,714
Answer: 9,000
6,000 + 3,000 = 9,000
27,684 + 15,739
43,423
7,436 − 2,758
4,678
3,004 − 1,728
1,276
A concert venue has 15,000 seats. 8,764 seats are filled.
How many seats are empty?
6,236 seats
Solve mentally:
7,002 − 2,998
Answer: 4,004
Possible strategy:
7,002 − 3,000 = 4,002
Since you subtracted 2 too much, add 2 back:
4,002 + 2 = 4,004
368,475 + 247,689
616,164
74,652 − 28,378
BONUS 20:
What inverse operation could you use to check?
46,274
BONUS
Addition!
46,274 + 28,378 = 74,652
A student solved:
5,002
- 2,738
-------
3,374
Is the student correct?
Answer: No.
Correct answer: 2,264
What should the student have done when they reached the zeros?
Move left until they find a place to regroup from, then regroup back across the zeros.
A wall needs 16,351 stones.
Builders have already placed 8,361 stones and have another 7,944 stones ready.
Do they have enough stones? If not, how many more do they need?
First:
8,361 + 7,944 = 16,305
Compare:
16,305 < 16,351
Answer:
No. They need 46 more stones.
Bonus if they can explain how to find 46 mentally.
Solve mentally and explain your adjustment:
8,002 − 3,997
Answer: 4,005
8,002 − 4,000 = 4,002
Since I subtracted 3 too many, I need to add 3 back.
4,002 + 3 = 4,005
A student solved:
48,576
+ 27,689
--------
65,165
What mistake did the student make? What is the correct sum?
76,265
The student did not correctly account for regrouping.
A student says:
723 − 458 = 335
Is the student correct?
Answer: No.
Correct difference: 265
20,000 − 8,635
Answer: 11,365
Require the team to explain how the regrouping travels across the zeros to earn the full $500.
A school fundraiser wants to raise $25,000.
On Monday they raised $8,475.
On Tuesday they raised $7,986.
How much more money do they need to reach their goal?
First:
8,475 + 7,986 = 16,461
Then:
25,000 − 16,461 = 8,539
Answer: $8,539 more