A manufacturing company uses an acceptance scheme on production items before they are shipped. The plan consists of two stages, that is, boxes of 25 are readied for shipment and a sample of 3 are tested for defectives. If no defectives are found the box is shipped.
What is the probability that
a) a box containing 3 defectives will be shipped?
b) a box containing only 1 defective will be sent back for screening?

Thank you very very much in advance!