in a box of 7 pencils , a total of 2 are defective . if a customer buys 3 pencils selected at random from the box , what is the probability that neither pencils will be defective ?
"first , there are 5 c 3 ways you can select 3 good pencils from 4 good ones . second , there are 7 c 3 ways you select 3 pencils from 6 ones in the box . then , the probability that neither pen will be defective is : 5 c 3 / 7 c 3 = 10 / 35 = 2 / 7 answer is d"
divide(divide(factorial(subtract(7, 2)), multiply(factorial(subtract(subtract(7, 2), 3)), factorial(3))), divide(factorial(7), multiply(factorial(subtract(7, 3)), factorial(3))))