Question

15 decreased by twice a number

Answer 1

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)

Answer 2

The correct equation would be:

15 - 2n

Hope this helps

-AaronWiseIsBae