The next generation of miniaturized wireless capsules with active locomotion will require

Question

two miniature electric motors to operate each capsule. Suppose 20 motors have been invented
but that, in spite of tests performed on the individual motors, 4 will not operate satisfactorily
when placed into a capsule. If the scientist wishes to construct two capsules, with two motors
each, find the probability that among the four randomly selected motors
a) all four operate satisfactorily;
b) three operate satisfactorily and one does not.

Answer 1

Refer to Example 12 of motors for miniaturized capsules,
but instead suppose that 20 motors are available
and that 4 will not operate satisfactorily, when
placed in a capsule. If the scientist wishes to fabricate
two capsules, with two motors each, find the
probability that among the four randomly selected
motors
(a) all four operate satisfactorily;
(b) three operate satisfactorily and one does not.