68 practice questions on Permutations and Combinations , sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Permutations and Combinations notes .
Start A B C B C A C A B 6 ordered outcomes (permutations); pairing AB/BA etc gives 3 combinations Counting tree for selecting 2 items from {A, B, C} without repetition: 3 x 2 = 6 ordered arrangements.
Easy - 20 questions Q3.
nPr represents:
A Number of unordered selections of r items from nB Number of arrangements of r from n distinct objectsC The product of n and r counted as a countD Sum of n and r treated as a combination countShow answer & explanation →
Q4.
nCr represents:
A Number of distinct ordered arrangements possible for r items chosen from nB Number of ways to select r items from n (order does not matter)C n raised to the power r, representing repeated outcomes with replacementD The numeric difference n minus r, treated loosely as a selection countShow answer & explanation →
Q14.
nPr = r! × nCr. True or false?
A TrueB False, claiming the identity does not actually holdC Valid just for the special case r=2D Valid just for the special case n=rShow answer & explanation →
Q17.
Multiplication principle: if 3 choices for first and 4 for second, total combinations:
A 3+4=7B 3×4=12C 3-4=?D 3<sup>4</sup>=81Show answer & explanation →
Medium - 20 questions Q27.
How many 4-digit numbers can be formed from 1-9 without repetition?
A 9×8×7×6 = 3024B 9!C 9P4D 9<sup>4</sup>Show answer & explanation →
Q28.
In how many ways can a team of 3 boys and 2 girls be chosen from 5 boys and 4 girls?
A 10 × 6 = 60B 5C3 × 4C2 = 60C 120D 5+4=9Show answer & explanation →
Q34.
The number of permutations of n things taken all at once when p are alike:
A n!/p!B n! × p!C p!/n!D (n-p)!Show answer & explanation →
Q35.
How many ways can 10 people be divided into two groups of 5?
A 10C5, the unsimplified combination countB 10C5 / 2 = 126C 10P5, treating order as significantD 252, double the actual correct countShow answer & explanation →
Q36.
Words from BOOK (all permutations):
A 12, from treating BOOK as having all distinct lettersB 24, computed as 4! without adjusting for the repeated letterC 12 (O repeated twice)D 4, counting only the distinct letters B, O, KShow answer & explanation →
Q40.
The middle term in the expansion of (1+x)<sup>2n</sup> is:
A (n+1)th term with coefficient 2nCnB nth term, taken as the halfway point before adjusting for indexingC (2n)th term, the very last term in the expansionD C(n,n), the coefficient of the final term x<sup>2n</sup>Show answer & explanation →
Hard - 28 questions Q41.
The number of ways to distribute n identical objects into r distinct groups (some may be empty):
A n!/(r!)B C(n+r-1, r-1)C C(n,r)D r<sup>n</sup>Show answer & explanation →
Q44.
Total number of permutations of n items with repetitions: n₁ alike, n₂ alike,..., nk alike (sum = n) =
A n!B n!/(n₁! n₂! ... nk!)C n!/(n₁+n₂+...)D n × n!Show answer & explanation →
Q45.
Number of ways to seat 5 couples in a row such that each couple sits together:
A 5! × 2⁵B 5! × 2⁴C 4! × 2⁵D 10!Show answer & explanation →
Q46.
In how many ways can 4 red, 3 blue, and 2 green balls be arranged in a row?
A 9!B 9!/(4!3!2!)C 4!3!2!D 24Show answer & explanation →
Q47.
Number of ways to select 3 from 6 where order matters for first 2 but not the third:
A 6P2 × 4B 6C3 × 2C 6P2 × 4C1D 6 × 5 × 4Show answer & explanation →
Q49.
The number of lattice paths from (0,0) to (m,n) moving only right or up:
A m+nB C(m+n, m)C m × nD m! + n!Show answer & explanation →
Q50.
Number of binary strings of length n with exactly k ones:
A n<sup>k</sup>B C(n,k)C k!/(n-k)!D 2<sup>n</sup>Show answer & explanation →
Q51.
Inclusion-Exclusion: |A ∪ B ∪ C| =
A |A|+|B|+|C|, leaving out all the overlaps between the setsB |A|+|B|+|C|-|A∩B|-|A∩C|-|B∩C|+|A∩B∩C|C |A|+|B|+|C|-|A∩B∩C|, subtracting just the triple overlap onceD |A∩B∩C|, taken as if it represented the entire unionShow answer & explanation →
Q56.
Total number of onto functions from A (m elements) to B (n elements) using inclusion-exclusion:
A n<sup>m</sup>, the count of all functions from A to B without restrictionB sum from k=0 to n of (-1)<sup>k</sup> × C(n,k) × (n-k)<sup>m</sup>C n! × C(m,n), mistakenly mixing a permutation count with a combinationD m<sup>n</sup>, swapping the roles of the domain and codomain sizesShow answer & explanation →
Q57.
The Stirling number S(n,k) counts:
A Arrangements of n objects that have exactly k fixed pointsB Partitions of n-element set into k non-empty subsetsC The number of k-element combinations chosen from n objectsD n choose k, the ordinary binomial coefficientShow answer & explanation →
Q59.
The generating function for combinations is related to:
A (1+x)<sup>n</sup> = sum nCr × x<sup>r</sup>B x<sup>n</sup>, the generating function associated with a single termC n! x, a linear function mistaken for a generating seriesD e<sup>x</sup>, the exponential generating function for permutations, not combinationsShow answer & explanation →
Q61.
The number of distinct arrangements of the letters of the word MISSISSIPPI is:
A 34650B 1663200C 1155D 83160Show answer & explanation →
Q64.
The number of ways to seat 5 boys and 3 girls in a row so that no two girls are adjacent is:
A 1440B 14400C 2880D 28800Show answer & explanation →
Q66.
The sum of all 4-digit numbers formed using each of 1, 2, 3, 4 exactly once is:
A 66660B 39996C 6660D 66600Show answer & explanation →