15) Marissa is 3 years older than her younger brother. Her older brother is twice as old as her younger brother. Her older brother is 18. How old are Marissa and her younger brother?

Marissa is 12 which means the younger bro is 9

I know the answer, but is there a special formula to work it out.

To find Marissa's age and her younger brother's age, we can use the information given in the problem. Let's start by assigning variables to the unknowns:

Let "x" represent the age of the younger brother.
Since Marissa is 3 years older than her younger brother, her age would be represented as "x + 3".
We are also given that the older brother is twice as old as the younger brother. Since the older brother's age is 18, we can represent it as "2x = 18".

To solve for "x", divide both sides of the equation "2x = 18" by 2:
2x/2 = 18/2
x = 9

So, Marissa's younger brother is 9 years old.

To find Marissa's age, we substitute the value of "x" into the equation "x + 3":
Marissa's age = 9 + 3 = 12

Therefore, Marissa is 12 years old and her younger brother is 9 years old.