The first sentence is always "a father is m times as old as his son". You choose the second sentence — its multiplier, its wording and the moment it describes — and the solver decodes it, brackets it and solves. The first three cases are three different second sentences that all describe the same father and son.
In 20 years he will be twice as old as his son
Father 40 · son 10
The most instructive move here: load the fourth case, then switch the wording control from “times more than” to “times as old as” without changing anything else. One word, and a 40-year-old father becomes a 10-year-old one.