Question

Five years ago, Linda was twice as old as Jacob. Now, Linda is 15 years old. Which equation can be used to find Jacob's current age, j?

Answer 1

If Linda is now 15 then 5 years ago the was 10 years old.
That means that Jacob was 5 years old 5 years ago (10-5=5)

Using $l$ to represent Linda's age and $j$ for jacobs age. 
jacob will always be 5 years younger than Linda so
$j=l-5$