​A boy told his sister, "I have sisters twice the number of brothers." The sister commented, "I have the same number of brothers and sisters." What is the number of girls in the family? ​

Correct option is B

Given:

A boy says, "I have sisters twice the number of brothers."

A sister says, "I have the same number of brothers and sisters." 

Concept used:

Solve this system of linear equations to find b and g

Solution: 

Let b be the number of boys and g be the number of girls in the family.

From the boy's statement: g = 2(b−1) (since he excludes himself while counting brothers).

From the sister's statement: b = g − 1 (since she excludes herself while counting sisters).

From the sister's statement: 

g = b +1 

Substitute g = b + 1 into the boy's equation:

g=2(b−1)

b+1 = 2 (b−1)

b+1 = 2b − 2

3 = b

Find g :

g = b+1: 

g = 3+1 = 4

There are 4 girls in the family.​