m.d.c (75;84)

This seems to be Spanish mínimo denominador común meaning minimum common denominator; or, more commonly in English, lowest (or least) common denominator.

prime factors of 75 are: 3, 5²

prime factors of 84 are: 2², 3, 7

Take the highest of each that occurs in both sets, namely:  2², 3, 5², 7 and multiply them all together to get:

$${{\mathtt{2}}}^{{\mathtt{2}}}{\mathtt{\,\times\,}}{\mathtt{3}}{\mathtt{\,\times\,}}{{\mathtt{5}}}^{{\mathtt{2}}}{\mathtt{\,\times\,}}{\mathtt{7}} = {\mathtt{2\,100}}$$

 So m.d.c(75, 84) = 2100

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Thanks for explaining that Alan.  :)