Math question

5 transistors are selected from 15 perfect transistorsand 5 defective transistors.

At least 3 transistors are perfect :
● Select 3 transistors from the 15 perfect ones (15C3)and 2 transistors from the 5 defective ones (5C2); OR
● Select 4 transistors from the 15 perfect ones (15C4)and 1 transistor from the 5 defective ones (5C1); OR
● Select 5 transistors from the 15 perfect ones (13C5).

Number of ways
= 15C3 × 5C2 + 15C4× 5C1 + 15C5
= 455 × 10 + 1365 × 5 + 3003
= 14378

A package of 20 transistors contains 15 that are perfect and five that are defective.
In how many ways can five of these transistors be selected so that at least three
are perfect?

A package of 20 transistors contains 15 that are perfect and five that are defective. In how many way scan five of these transistors be selected so that at least three are perfect?

2015-01-08 00:40:16 補充：
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