Adding/subtracting wrong amount for bounds H
"The Half-Step Slip"
The Mistake in Action
A mass is 250g to the nearest 10g. Find the bounds.
Wrong: LB = $250 - 10 = 240$g, UB = $250 + 10 = 260$g
Why It Happens
Students subtract/add the whole rounding interval instead of half of it.
The Fix
The error is half the degree of accuracy.
For rounding to nearest 10:
- LB = $250 - 5 = 245$g
- UB = $250 + 5 = 255$g
Why half? Values from 245 to 254.999... all round to 250.
| Degree of accuracy | Error is |
|---|---|
| Nearest 10 | ±5 |
| Nearest 100 | ±50 |
| Nearest integer | ±0.5 |
| 1 d.p. | ±0.05 |
Spot the Mistake
Can you identify where this student went wrong?
250g to nearest 10g
LB = $250 - 10 = 240$g
Click on the line that contains the error.
Related Topics
Learn more about the underlying maths: