Upper and lower bounds

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.

Substituting a mixture (e.g. an upper bound for one quantity and the rounded value for another) loses marks.

Putting the given (rounded) numbers into the formula and then "finding bounds" of the result scores nothing.

To MAXIMISE $a-b$ or $a \div b$, take the upper bound of $a$ but the LOWER bound of $b$: $\text{UB}(a-b) = \text{UB}(a) - \text{LB}(b)$. Using $\text{UB}(b)$ makes the result too small. Difference and quotient mix the bounds; sum and product do not.

The bound is half of the rounding UNIT, not half the value. To the nearest $10$ m, $80$ m gives $75 \le d < 85$ (half of $10$); to $1$ d.p., go $0.05$ either side. Using $\pm 0.5$ whatever the stated accuracy is a frequent slip.