Answer
There are four boys and six girls in the family.
If there were B boys, and G girls we know that Billy has twice as many sisters as brothers, so 2 x (b - 1) = g.
And his sister, Frankie, has one more sister than she has brothers, so g - 1 = b + 1.
2(b - 1) = g [1]
g - 1 = b + 1 [2]
Using [1] in [2] gives:
2(b - 1) - 1 = b + 1
Expanding the bracket gives:
2b - 2 - 1 = b + 1
Simplifying the left hand side gives:
2b - 3 = b + 1
Add 3 to each side gives:
2b = b + 4
Subtract b from both sides gives:
b = 4
Using b = 4 in [1] gives:
2(4 - 1) = g
6 = g
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