Hi!
Your answer is [tex]f-15[/tex]
"Decreased by" implies subtraction, because you are decreasing its value. If you are decreasing a number by a value, you are subtracting that value from said number.
Therefore, if [tex]f[/tex] is decreased by [tex]15[/tex], you are subtracting [tex]15[/tex] from [tex]f[/tex].
Which gets you [tex]f-15[/tex]
For another example, see:
https://brainly.com/question/19437083
