Clock that requires math to read the time

[rprosperi]

I've never encountered the ":" symbol for division (in USA), except in several prior discussions here in MoHPC, where it is consistently mentioned as the most common symbol used in some countries in Europe, while not in others, though I don't recall which were which.

From Wikipedia:

Quote: a ÷ b

This form is infrequent except in elementary arithmetic. ISO 80000-2-9.6 states it should not be used. The obelus is also used alone to represent the division operation itself, as for instance as a label on a key of a calculator. The obelus was introduced by Swiss mathematician Johann Rahn in 1659 in Teutsche Algebra.[10]:211

a : b

In some non-English-speaking countries colon is used to denote division.[11] This notation was introduced by Gottfried Wilhelm Leibniz in his 1684 Acta eruditorum.[10]:295 Leibniz disliked having separate symbols for ratio and division. However, in English usage the colon is restricted to expressing the related concept of ratios.