My kids really liked a tablet game called DragonBox, which, according to its promotional material, “secretly teaches algebra to your children.” Kind of like the way you can secretly feed kale to your children by grinding it up and hiding it in a meatball. But what DragonBox actually teaches are the rules of algebra: that you’re allowed to add the same symbol to both sides, that multiplying a sum by a symbol requires you to multiply each summand by the same symbol, and so on. Getting these rules in muscle memory is what you need if you want to get fluent in algebraic computations.