This family puzzle takes a little algebra, but the wording gives you everything you need.
Suppose there are b boys and s girls.
For a boy, the number of sisters is s, while the number of brothers is b − 1. The puzzle says those are equal, so:
s = b − 1
For a girl, the number of brothers is b, while the number of sisters is s − 1. The puzzle says she has twice as many brothers as sisters:
b = 2(s − 1)
Substituting s = b − 1 gives b = 4 and s = 3.
So there are 7 children altogether.
Answer: 7 children — 4 boys and 3 girls.
