Question 153671
Bob is 20 years older than Ben. In 10 years, Bob will be twice as old as Ben. How old is each now?
.
Let x = age of Ben
then because "Bob is 20 years older than Ben" 
x+20 = age of Bob
.
Our equation is derived from: "In 10 years, Bob will be twice as old as Ben."
(x+20) + 10 = 2(x+10)
x + 30 = 2x + 20
30 = x + 20
10 years = x (age of Ben)
.
x+20 = 10 + 20 = 30 years (age of Bob)