[Mike Haven]

no

there are 17 boys (13 girls)

there may be, say, 2 redheaded boys, so of the 4 redheads 2 are girls

so there are 19 boys or redheads (in this case)

or, take 2 redheaded boys from 17 boys = 15, and add the 4 redheads = 19 boys or redheads