The rule is simply a convention to make sure everyone comes up with the same answer. However, this rule doesn't always work. The best way to avoid confusion is to isolate chunks of arithmetic in brackets. Do the shit in brackets before you do the shit that links them.
Example. 2x4+2=10 according to the above rules. However, maybe I meant that I need two sets of lumber, each with a 2 foot and a 4 foot long board (six feet of board per set) and I want to know the total length of board that I need. The answer would not be 10 feet, it would be 12 feet. The way to avoid ambiguity is this: 2X(4+2)=12. Do the shit inside the brackets first, and you get the right answer. I always use brackets because even spreadsheets can get up to mischief if you don't know exactly what the invisible sequence rules are. If the calculation is not simple, it is not obvious whether it is the right answer given the intended purpose of the calculation.
I'm sure that all my stoner buds were on the edge of their chairs waiting for this.
I do get carried away.
Off to the next couple instalments of Vikings. More hacking and slashing.