[evad]

"Try that one with 2/3 + 1/7"

Looks like 17/21 to me...

Mee Too..but not by the professor's method

[Principled]

It works, of course. It is indeed the same process as "getting a common denominator".... without supporting reasoning - valid nevertheless.

The 2/3 is different than previous examples b/c the numerator is a number other than 1.

The professor's process still works, but because the reasoning is omitted, most didn't know that the numerators are multiplied by the other denominator before they're added - when the numerators were 1 it didn't matter.