To solve this, we simply need to break down and comprehend the words and form our equation.
"45 decreased by"
This shows that we are going to be subtracting 45 by something.
45-
"twice a number"
The "twice" implies that it is going to be 2 multiplied by something, and "a number" represents our variable, n.
45-2n
Using the logic above, we can see the equation to represent this phrase is 45-2n.