Whenever a verbal phrase says "a number decreased by another number" the number that goes first is the minuend and the one that follows is the subtrahend. And the expression asks for 2 times said number so it's 15-2a (or any other variable)
The correct equation would be:
15 - 2n
Hope this helps
-AaronWiseIsBae
6.2m questions
8.2m answers