If we work with the given information we'll be able to figure it out:
There are 30 boys and 1/2 are wearing sweaters, which means 1/2 aren't wearing sweaters. 1/2 of 30 is 15, so 15 boys are wearing sweaters and 15 aren't.
There are 18 girls and 1/3 are wearing sweaters, which means 2/3 aren't wearing sweaters. 1/3 of 18 is 6 and 2/3 of 18 is 12, so 6 girls are wearing sweaters and 12 aren't.
15 boys and 12 girls aren't wearing sweaters