A variable associates a name with a value of a particular type. There are two main ways to create variables in Swift. `let` and `var`. To declare constants use the `let` keyword. To declare mutable variables use the `var` keyword.
In this case the variable `daysInAWeek` should be a constant because there are only seven days in a week, while the variable `amountOfMoney` should be a var because the amount of money in one's account changes.