Based on the given mathematical statement, it can be rewritten into a mathematical equation to simplify and to be solved easily. The statement, "When one is subtracted from three-tenths of a number" corresponds to the equation, 3x/10 - 1, that is assuming the number is represented by the variable x. This is equal to one-fifth of the number. In other words,
3x/10 - 1 = x/5
x = 10