If a daily supplement contains 310 mg of magnesium, does this intake exceed the 100 mg/day major-mineral cutoff and by what factor?

Yes — but only by a negligible amount of 0.31 times the cutoff because you should divide the cutoff by the intake to obtain the appropriate ratio for dietary counselling.

No — magnesium is always classified as a trace mineral regardless of intake level, so numerical comparisons to a cutoff are irrelevant for classification.

No — it is below the cutoff because 310 mg is less than 1,000 mg which is the more appropriate major-mineral threshold used in clinical practice for magnesium.

Yes — it exceeds the cutoff; 310 ÷ 100 = 3.1 times the cutoff.