a box of 20 transistor is known to contain 4 that are defective. the transistor are to be tested, one at a time, until defective are identified. denote n1 the number of test made until the first defective is spotted and n2 the number of of additional test until the second detective is spotted. find the joint probability mass function (4,4)(2,2)(2,1)(1,2)(3,2)(4,2)

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