Answer:
owner = 10 years old
mouse = 3 years old
Explanation:
Let's say the mouse is currently m years old and the owner is currently o years old.
m + o = 13
In three years, a pet mouse (m+3) will be as old (=) as his owner was 4 years ago (o-4)
m+o=13
m+3=o-4
subtract o from both sides in the first equation to solve for m
m = 13 - o
plug 13-o for m in the second equation to isolate one variable in one equation
13-o+3=o-4
16-o=o-4
add o to both sides to put all the variables on one side
16=2o-4
add 4 to both sides to isolate the variable and its coefficient
20 = 2o
divide both sides by 2 to isolate o
o = 10
m = 13 - o
m = 3