2.6.21. A th ree-.digi t number is to be formed from the digi ts 1 through 7. with no digit being
used more than once. How many such n umbers would be less than 289?
2.6..22. Four men and four women are to be seated in a row of chairs num bered 1through 8.
(a) How many total arrangemen ts are possi ble?
(b) How many arrangemenrs are possible if the men are required to sit i n alternate chairs?
2.6..23. An engineer needs to take 1h ree tech nical electives sometime during his final four semesters. The three are to be selected from a list of ten. In how ma ny ways can he schedule those classes, assuming that he never wants to lake more than one technical elective in any given term?
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