|Spoiler Inside: Solution to Puzzle||SelectShow>|
If you try to solve this by working with the number of brothers and sisters, you will run into the problem that this is relative. Susan and Tom have a different number of brothers and sisters.
Instead, work on figuring out how many siblings of each sex there are. Let f and m be these.
From Susan’s perspective: m = 2(f – 1).
These equations simplify to m – 2f = –2 and –2m + f = –2. Solving this system gives m = 2 and f = 2.
Susan is one of two sisters so she has one sister; she has two brothers (twice as many as one).
Similarly, Tom is one of two brothers so he has one brother; he has two sisters (twice as many as one).