Her salary 4 years ago would have been $30,000.
If 45,000 is 15,000 less than double her previous salary, we must first add 15,000 to 45,000 to find what double her salary was.
45,000+15,000=60,000.
Since this is twice her salary, we divide this by 2.
60,000/2= 30,000.
So, her salary 4 years ago would have been $30,000.
I hope this helps!