Using wrong bounds for subtraction H
"The Subtraction Swap"
The Mistake in Action
Find the minimum value of $a - b$ where $a = 8.5$ and $b = 3.2$ (both to 1 d.p.).
Wrong: Minimum = $8.45 - 3.15 = 5.30$
Why It Happens
Students use LB − LB for minimum, not realising that subtracting a smaller number gives a larger result.
The Fix
For minimum of $a - b$:
- Use smallest $a$ → LB of $a$
- Use largest $b$ → UB of $b$
Minimum = LB(a) − UB(b) = $8.45 - 3.25 = 5.20$
Think about it: To get the smallest difference, make the first number as small as possible and the second as large as possible.
Spot the Mistake
Can you identify where this student went wrong?
Minimum of $a - b$
$= 8.45 - 3.15 = 5.30$
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: