[Mike Haven]

i didn't see his article

i gave an example to show that his instructions were correct

his instructions are correct:

"There are 30 children in your child's class. How many are either boys or redheads? If I tell you that there are 13 girls and four redheads, that doesn't mean the answer is 17...To get the right answer, you would in fact have to subtract the number of redheaded boys from 17."

if i tell you there are 13 girls in a class of 30 there are 17 boys

if i tell you there are 4 redheads they may be boys or girls

if there are, say, two redheaded boys then you have to subtract this number from the 17 boys = 15

then add the 4 redheads = 15 + 4 = 19

otherwise you are double-counting the redheaded boys