The BODMAS mathematical rule.

[GoSeigen]

But in this instance that isn't the "howler" modellingman was alluding to. Rather it was the obvious mistake, in the working out, of describing

6/2*3 = 3/3.

Which should instead have been(?) described as either: 3*3 = 9 or 6/6 = 1(?)

Not if we are consistent in reading the "/" as the horizontal line separating numerator and denominator. In that case 2*3 forms the denominator. Here is 6/2*(2+1) =6/2*3=3/3 =1 laid out as Dod is imagining it:

.  6           6         3
-------  =  -------  =  ---   = 1
2*(2+1)       2*3        3

Note 1, that when laid out as above, it is not necessary to surround the denominator with brackets: it is evaluated before the whole fraction is evaluated even without the brackets.
Note 2, that I don't think that doing things this way is a good idea, it's just that I sympathise with the [Dod] interpretation if the context is either unknown or implies it.

...

...but you could have done 6/2*(2+1) =6/2*3=3/3 =1 in strict accord with the BODMAS rules.

Then the 3/3 simply results from dividing by two in both the numerator and denominator.

Oh right!

Hopefully you see it now... can be hard to step in others' shoes sometimes!!

GS