The way it was explained to me, the value of $a is modified with all the parenthetical calls, and then the result is added. So the actual calculation would end up being:
$a = 30
then $a = 35 (after the +=5)
then $a = 25 (after the -=10)
then $one = $a + $a, or (25)+(25) = 50, our answer.
I knew the code wasn't intuitive; it was presented as a specifically nonintuitive snippet in order to show the problems inherent in unclear code, in the example I was looking at. I got curious as to the result, and fretted over it until Zaxo laid it out for me.