My answer would be:
--- to solve a problem algebraically, you use an equation, involving one or more unknowns;
--- to solve a problem arithmetically, you don't write an equation.
Example:
If 5 dogs eat 10 pounds of dog food , how many dogs (same size) would it take to eat 40 pounds of dog food?
Algebraically: 5/10 = x/40 ---> cross multiply ---> 10x = 200 ---> x = 20 dogs.
Arithmetically: Think this way: if 5 dogs ate 10 pounds, each dog ate 2 pounds, and 40 / 2 = 20 dogs.