Ten different letters of an alphabet are given. 2 of these letters followed by 2 digits are used to number the products of a company. In haw many ways can the products be numbered?
If P (2n+1, n-1): P (2n-1, n) = 3:5, find n.
A polygon has 20 diagonals. How many sides does it have?
A box contains 5 red and 4 blue balls. In how many ways can 4 balls be chosen such that there are at most 3 balls of each colour?
Six points lie on a circle. How many quadrilaterals can be drawn joining these points?
In how many ways can 15 students be seated in a row such that the 2 most talkative children never sit together?
