How do i turn the variable into a number?

Sometimes you have a data set with a variable that appears to be a numeric variable, perhaps because it has numeric values, but is really a string variable. Because you cannot perform most statistical operations on a string variable, you may want to turn the string variable into a numeric variable. Consider the following data set.

The coefficient of the expression is 0.25, and it represents the late fee for every day the book was overdue