"Twice more than (two times more)" vs "Twice as much"

A common mistake by my student is "twice more than", they often mistake it as "twice as much as". Actually they are different:

•   Twice as much = 2 X the base
•      Twice more than (Two times more) = 2 X the base + the base

Here is an example:

Tom's present age is twice more than Sam's age years ago. Today, Sam's age is Tom's age the same number of years ago. How old is Tom now if the sum of their present age is 30?

Solution:

In this question, "Tom's present age is twice more than Sam's age years ago", means Tom's present age is 3u, Sam's age years ago is 1u.