Find n if ^n-1P₃ : ⁿP₄ = 1: 9.

15

Find the number of 4-digit numbers that can be formed using the digits 1,2,3,4,5 if no digit is repeated. How many of these will be even?

16

From a committee of 8 persons, in how many ways can we choose a chairman and a vice chairman assuming one person cannot hold more than one position?

18

Find r if :  (i) 5P_r = 2 6P_r-1   (ii) 5P_r = 6P_r-1

19

How many words, with or without meaning, can be formed using all the letters of the word ‘EQUATION’, using each letter exactly once.