A rounded value has a range
A value given to a degree of accuracy could really be anything in a range half a unit either side of it.
Choose bounds to make the result biggest or smallest
For a sum/product use both upper (or both lower); for a difference/quotient mix them: UB(a−b)=UB(a)−LB(b), UB(a÷b)=UB(a)÷LB(b).
Drawn from real examiner reports.
Using 28.499… instead of 28.5 as the upper bound
The upper bound is the exact halfway value (28.5), not a string of 9s like 28.499 — terminating decimals just below it are not accepted.
Mixing upper and lower bounds in one formula
Substituting a mixture (e.g. an upper bound for one quantity and the rounded value for another) loses marks.
Substituting the rounded values instead of the bounds
Putting the given (rounded) numbers into the formula and then "finding bounds" of the result scores nothing.
Give the answer to the accuracy asked
If asked for an answer to 3 s.f., round at the end — don't leave 12.750… unrounded.
A measurement rounded to the nearest lies within of the stated value:
12 cm to the nearest cm — bounds?
A length cm, measured to the nearest centimetre. Write down the lower and upper bounds of .
