Convert to improper
Write both mixed numbers as improper fractions before touching the sign.
AQA 8300 · N2/N8 · Signed mixed numbers
Signed mixed numbers: converting before subtracting
The conversion mark. The method mark is for converting to improper fractions before the sign is resolved, not for the final value.
Reviewed family · Studara-authored items · No grade-uplift claim
The lost mark
One answer. Several places the method can fail.
Studara does not reduce the response to right or wrong. The reviewed contract checks these states independently.
Common denominator
Rewrite both over a common denominator and combine the numerators.
Return to a mixed number
Simplify and convert back, carrying the sign onto the whole number correctly.
Why it goes wrong
The diagnosis stays uncertainty-safe.
A wrong response can support a specific measured diagnosis. A blind multiple-choice diagnostic records only a possible cause until later working provides stronger evidence.
01
The error
Subtracting the whole parts and the fractional parts separately, which produces a negative fraction attached to a positive whole — a quantity that is not a mixed number at all.
02
Why it survives revision
It gives the correct answer whenever the first fraction is larger than the second, which describes most worked examples. The method is not partially right; it is right on a subset the practice set happens to be full of.
The worked repair
Repair the smallest broken step.
Teaching is not evidence. This one move is the repair; the proof checks begin after it disappears.
Convert both to improper fractions first. Resolve the sign after, never during.
What happens after the repair
Correction is the start, not the result.
Immediate
Asked again straight away, on a different question
The repair leaves the screen and a new authored item checks the method unassisted. Being taught something is never evidence that it was learned, so this is the first stage that counts at all.
Delayed
Asked again after at least 1 day
A parallel form of the same method, unassisted, with the repair no longer on screen. A retest in the same session does not count.
Transfer
Asked again when the surface changes
The transfer form makes the first number the negative one, so the sign has to be carried through the conversion rather than assumed at the end.
Exam
Asked again under exam conditions
Timed, carrying the mark tariff, in an exam-like response format and aligned to the specification. Any one of those missing and the evidence is refused.
REVIEWED BOUNDARY
This page covers one reviewed family: N2/N8 · Signed mixed numbers. Expose the sign, improper-fraction conversion and every common-denominator state. Other families within the same GCSE Maths topic are outside this reviewed journey, and Studara does not apply the proof label to material that has not been reviewed. No grade-uplift claim is made anywhere on this site.
Straight answers
Before you use this guide.
Can I subtract the whole numbers and fractions separately?
Only when the first fraction is larger than the second. When it is not, the method produces a negative fraction attached to a positive whole, which is not a mixed number. Converting to improper fractions first always works.
What do I do when the answer is negative?
Resolve the sign after combining, not during. Convert to improper fractions, combine, then attach the sign to the whole result.
Does Studara guarantee a higher grade?
No. Studara has not run a controlled outcome study and publishes no grade-uplift claim. What has been measured, what is withheld, and the metric that failed its bar are all on the public evidence page.
Turn this method into evidence