Since their is a rat and a mouse, we need to figure out their ages.
Ok, so we know that the rat is x years old, we also know that the mouse is 15 years YOUNGER than the rat.
Let us set up an equation.
We know that younger usually refers to subtraction.
And we know that x is our variable, and that we need to solve for the mouses age.
m = mouse
x= rats age
m = x - 15
Our final answer: The equation is m= x - 15