We can solve these types of problems by simply breaking down each part.
"The sum of a number and"
From this, we can conclude that it has to be x+ something, because "a number" represents "x" and "sum of" represents "+."
x+
"Twice that number"
We can conclude that this means "2x" because "twice" means 2 and "that number" means it will be the same variable as earlier.
x+2x
"is 45"
"Is" almost always means "=," and "45" is the value that it is equal to.
x+2x=45
Using the reasoning above, we can see that A) x+2x=45 is the correct answer.
