first find 5% of the girls. 5% = .05

so multiply 120*.05, which equals 6

so 6 girls leave

now there are 120-6, or 114 girls and 57 boys

the total number of students is 114+57 = 171

the percentage of boys is given by 57/171 = 1/3 = .3333

so approximately 33.33% or 33% are boys