s=sisters
b=brothers

Subtract Ana from sisters = number of brothers
s-1=b

When we subtract Andrew from brothers (b-1)
This gives us twice as many sisters as brothers 2(b-1):b-1

2(b-1)=s

plug the equation for b into the equation for s
2b-2=s
2(s-1)-2=s
2s-2-2=s
2s-4=s
s=4

now we have the number of sisters. plug 4 into the equation for b.
b=3

Total = 7

Let B = number of boys that Emma has
Let G = number of girls that Emma has

Ana is a girl and has the same number of brothers as sisters.
B = the number of brothers Ana has
Since Ana is one of Emma's girls, we know that G-1 = the number of sisters Ana has (we cannot include Ana in this count, since Ana is not her own sister)
We can write: B = G-1

Andrew is a boy and has twice as many sisters as brothers.
G = the number of sisters Andrew has
Since Andrew is one of Emma's boys, we know that B-1 = the number of brothers Andrew has
We can write: 2(B-1) = G

We now have the following system:
B = G-1
2(B-1) = G

Simplify the bottom equation to get:
B = G - 1
2B - 2 = G

Take the blue equation and replace B with G-1 to get: 2(G-1) - 2 = G
Expand: 2G - 2 - 2 = G
Solve: G = 4

When we plug G = 4 into either equation and solve for B, we get B = 3
So, B+G = 3+4 = 7
So, Emma has 7 children (3 boys and 4 girls)

Answer: D

Cheers,
Brent

I think sketching a tree is quicker:
emme has:
anna andru
sis bro
sis bro
sis

total is 7 children