in a box of 9 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 7 c 3 ways you can select 3 good pencils from 4 good ones . second , there are 9 c 3 ways you select 3 pencils from 6 ones in the box . then , the probability that neither pen will be defective is : 7 c 3 / 9 c 3 = 35 / 84 = 5 / 12 answer is a
divide(divide(factorial(subtract(9, 2)), multiply(factorial(subtract(subtract(9, 2), 3)), factorial(3))), divide(factorial(9), multiply(factorial(subtract(9, 3)), factorial(3))))