My neighbor has seven children. Every brother has equal no. of brothers and sisters, but each sister has twice as many brothers as sisters. How many brother & sister?

method 1:

Since the boys have as many brothers as sisters, there must be 1 boy more than the number of
girls.
There must be 4 brothers and 3 sisters to fulfill the requirements.

each boy has 3 brothers and 3 sisters( brother has equal no. of brothers and sisters)

each girl has 4 brothers and 2 sisters (each sister has twice as many brothers)

Hence there is 4 brothers and 3 sisters in family.

method 2:

Total no of brothers and sisters are 7.
B+S=7...........(I)
Since the boys have as many brothers as sisters, there must be 1 boy more than the number of
girls and each sister has twice as many brothers as sisters
B =2(S-1).......(II)
substituting value of in equation (I)
B+S=7
2(S-1)+S=7
2S-2+S=7
3S=9
S=3
Now substituting value of S in equation (II)
B=2(S-1)
B=2(3-1)
B=2*2
B=4
Hence there is 4 brothers and 3 sisters in family.