3 brothers and 4 sisters. (b = brothers and s = sisters; "A girl has as many brothers as sisters" --> b = s - 1; "Each brother has only half as many brothers as sisters" --> b - 1 = 1/2 * s; Substituting the first equation into the second equation --> (s - 1) - 1 = 1/2 * s; Simplifying that --> s - 2 = 1/2 * s; Multiplying both sides by 2 --> 2s - 4 = s; Subtract "s" from both sides, add 4 to both sides --> s = 4; Substituting into the first equation --> b = 4 - 1 --> b = 3)