Pick a phrasing for each of the three links and pick the anchor. The solver writes the links first, finds the anchor second and climbs the chain third — the same order you should use on paper. The first two cases are the same four people described in completely different words.
In 10 years P will be 50; twice, 5 older, half
P 40 · Q 20 · R 15 · S 30
Load the second case and compare it with the first: three rewritten sentences, identical ages. Then set the anchor to the total and watch the method change shape without changing answer. To see a guard fire, use the small anchor with the 20-year link.