If x=girls,
then we have 3x boys.
We know that all children are boys+girls, so x+3x, which is 4x and 4x is 48, so one x is 12.
So there are 12 girls and 36 boys.
5,6
55,56
65,66
555,556,565,566
655,656.665.666
Looks like there are 14 numbers that use only the digits 5 or 6 or both 5 or 6 that are under 1000.
The correct answer to your question is 8.
19 comes next in the series, followed by two 21s.
