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4. A shipment of 20 cellphones contains 3 defective units. In how many ways can a company purchase four of these units and receive all working units? *

Sagot :

Answer:

Assuming random selection, no prior knowledge, order does not matter.

a) 22*21*20*19 or 22 C 4 - which is 7315 (note: total combinations , good or bad would be 25 C 4 or 12650 )

b) 2 bad units ( 3*2 or 3 C 2 - 3) times 2 good units ( 22*21 or 22C2 - 231) or 693

c) Same as asking how many ways to get 2 or less bad units. That would be 0 bad units (answer to a) plus 2 bad units (answer b) , plus 1 bad unit ( 3C1 * 22C3 or 1543), which together equal 9551

Step-by-step explanation:

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