
Discrete Mathematics
Problem Sheet 3
Principle of Inclusion - Exclusion:
1. A certain computer center employs 100 computer programmers. Of these
47 can program in FORTRAN, 35 in Pascal and 23 can program in both
languages. How many can program in neither of these 2 languages?
2. Suppose that, in addition to the information given in 1, there are 20
employees that can program in COBOL, 12 in COBOL, 12 in COBOL
and FORTRAN, 11 in Pascal and COBOL and 5 in FORTRAN, Pascal,
and COBOL. How many can program in none of these 3 languages.
3. Find the number of integers between 1 and 1,000 inclusive that are divisible
by none of 5,6, and 8. Note that the intersection of the set of integers
divisible by 6 with the set of integers divisible by 8 is the set of integers
divisible by 24.
4. Eight people enter an elevator at the first floor. The elevator discharges
passengers on each successive floor until it empties on the fifth floor. How
many different ways can this happen?
5. How many arrangements are there of MISSISSIPPI with no pair of con-
secutive letters the same?
6. Suppose that a person with 10 friends invites a different subset of 3 friends
to dinner every night for 10 days. How many ways can this be done so
that all friends are included at least once?
7. How many arrangements are there of 3.a’s, 3.b’s and 3.c’s
(a) without 3 consecutive letters the same?
(b) having no adjacent letters the same?
8. Use the principle of inclusion exclusion to count the number of primes
between 41 and 100 inclusive.
9. How many 6-digit decimal numbers contain exactly three different digits?
Recurrence Relations:
1. Find the coefficient of X16 in (1 + X4+X8)10.
2. Find the generating function for the number of r-combinations of {3.a,
5.b, 2.c}.
3. Build a generating function for determining the number of ways of making
change for a dollar bill in pennies, nickels, dimes, quarters, and half-dollar
pieces. Which coefficient do we want?
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