Write an algebraic expression for 15 decreased by twice a number

An algebraic expression is, obviously, an expression with a variable.

"A number" would represent a variable, which, in this case, we'll use y.

So, 15 decreased by twice x.
15 - y x 2 = 

I hope this helped :)

15-2x= that would be the algebraic equation. Remember that decrease usually means minus. A number is always represented by a variable, and then anything that by is usually a multiplication or division depending on how its stated.