girls= g
boys=b
50=g+b
Now we know that 30% of the girls are younger than 12. That's already half to answer...
We also know that 40% of the boys are OLDER than 12, which leaves 60% of the boys YOUNGER than 12.
This makes us know that 30%+60% out of 50 kids are younger than 12
30%+60%=90%
90% of the kids are younger than 12
90% of 50 is 45
ANSWER: 45 kids are younger than 12