It seems that putting \column before itemize of 'c', forces 'c' together with this content. Repeating this step produces what I want:

\startexercici[exercici:operacions-amb-fraccions-2]{multiplicació i divisió} Opereu: \startitemize[a, columns, three] \item \startformula 3 \cdot \frac{5}{3} \stopformula \item \startformula \frac{3}{4} \cdot \frac{2}{5} \stopformula \column \item \startformula \frac{1}{3} \cdot \frac{3}{4} \div \frac{1}{5} \stopformula \item \startformula 5 \div \frac{7}{10} \cdot 7 \div \frac{2}{5} \stopformula \column \item \startformula \frac{1}{2} \cdot \frac{1}{2} \stopformula \item \startformula \frac{3}{2} \div \frac{2}{3} \div \frac{5}{4} \stopformula \column \item \startformula \frac{2}{3} \colon \frac{2}{5} \stopformula \item \startformula \frac{3}{4} \colon \frac{1}{2} \cdot 6 \stopformula \item \startformula \frac{2}{3} \colon \frac{3}{4} \cdot \frac{1}{2} \stopformula \stopitemize \stopexercici

@fv leung said I have to replace frac for dfrac, but it mess up the following content in my book.

Thanks all,

On Thu, 24 Feb 2022 23:04:43 +0000 Bruce Horrocks ntg@scorecrow.com ha escrit:

...

On 24 Feb 2022, at 22:10, Xavier B. via ntg-context ntg-context@ntg.nl wrote:

Even if I change \startexercici[exercici:operacions-amb-fraccions-2]{multiplicació i divisió} Opereu: \startitemize[a, columns, three]

for

\startexercici[exercici:operacions-amb-fraccions-2]{multiplicació i divisió} Opereu: \startitemize[a, columns]

the 'c' jumps.

Isn't ConTeXt capable of handle that?

Please see the attached PDF.

The green line shows how much space is needed to fully print item (b). Item (c) will need the same amount of space, shown by the red line. However, this would take you into the footer area (shown by the blue arrows), so item (c) is split: the item label fits into the space at the bottom of the column and so is printed there; the fraction doesn't fit and so is moved to the next column.

The solution to your problem would be some way of forcing the item body to stay with the label. I don't know what that command is - hopefully someone here does know.

— Bruce Horrocks Hampshire, UK