Rounding Boundaries Worksheet
Math Mastery Lab • Place Value
Name: Date:
Rounding Boundaries
Find the smallest and largest whole numbers that round to a target value.
Part 1: Nearest Ten
The Halfway Boundary Secret
Smallest Number: The lower halfway point rounds UP to the target.
(Rule: Target − 5)
Largest Number: The number just before the upper halfway point rounds DOWN.
(Rule: Target + 4)
1
What is the smallest whole number that rounds to 70 when rounded to the nearest ten?
Fill in the missing benchmark & halfway values on the number line: Target: 70
60 70 (Target) 80 fill 65 fill 75
Step A: Halfway mark What number is exactly halfway between 60 and 70?
Answer: _____
Step B: Rounding Rule When a number ends in 5, does it round UP or DOWN?
Circle: UP / DOWN
Final Solution
Smallest number that rounds to 70:
2
What is the largest whole number that rounds to 140 when rounded to the nearest ten?
Mark the target (140) and show why the largest number cannot be 145: Target: 140
130 140 (Target) 150 fill 145
Step A: The Upper Cutoff What number is halfway between 140 and 150?
Halfway: _____
Step B: The Trap! Why does 145 not round down to 140?
Because 145 rounds UP to .
Final Solution
Largest number that rounds to 140:
Turn to Page 2 for Rounding to the Nearest Hundred & the Challenge Problem
Rounding Boundaries • Part 2: Nearest Hundred
Page 2 of 2
Hundred's Rule: Look at the tens digit! The halfway mark is always 50.
Smallest = Target − 50 | Largest = Target + 49
3
What is the smallest whole number that rounds to 400 to the nearest hundred?
300 400 (Target) 500 fill 350
Halfway Value: Number halfway between 300 & 400: _____
Round Check: Does 349 round to 400?
Yes / No (Rounds to: ____)
Smallest to round to 400:
4
What is the largest whole number that rounds to 800 to the nearest hundred?
700 800 (Target) 900 fill 850
Next Halfway Mark: Halfway point between 800 & 900: _____
The 1-Less Strategy: What whole number comes immediately before 850? _____
Largest to round to 800:
5 Challenge
What is the largest whole number that rounds to 1,000 to the nearest hundred?
900 1,000 (Target) 1,100 fill 1,050
Identify the Upper Cutoff: Halfway point between 1,000 and 1,100: ______
Check Boundary: Does 1,049 round to 1,000 or 1,100?
Answer: ______
Largest to round to 1,000:
The Rounding Boundary Master Rules
When Rounding to Nearest 10:
• Smallest: Subtract 5 from target (Target − 5)
• Largest: Add 4 to target (Target + 4)
When Rounding to Nearest 100:
• Smallest: Subtract 50 from target (Target − 50)
• Largest: Add 49 to target (Target + 49)
