In how many ways can 5 girls be seated in a row so that two girls Ridhi and Sanya are:
(a) always together (b) never together
Number of digits = 10 (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
Number of digits used = 4
The digits are not to be repeated.
Number of permutations for box 1 = 9 (zero cannot be placed in box 1)
We have 9-digits left (no repetition is allowed) and three boxes.
Number of permutations
By fundamental principle of counting, the number of permutations, when a digit is not repeated
= 9 x 504 = 4536.
(b) there is no restriction as to the number of prizes that a boy may get.
(c) no boy sets all prizes.
There are 8 students appearing for an examination, of which 3 appear in mathematics paper, and 5 in other different subjects. In how many ways can they be seated if
(a) all the students appearing for mathematics paper are together.
(b) the students appearing for mathematics paper are not all together.
(c) no two students appearing in mathematics paper are seated together ?