### 18. Ten different letters of an alphabet are given. Words with five letters are formed from these given letters. Then the number of words which have at least one letter repeated is

### 19. If sides AB, BC, CA of a triangle ABC have 3, 4 and 5 interior points respectively on them, then total number of triangles that can be constructed by using these points as vertices, is

### 20. A student can take one of four Mathematics sections and one of five English sections. The number n of ways he can register for the two courses, is

### 21. There are (n + 1) white and (n + 1) black balls each set numbered 1 to n + 1. The number of ways in which the balls can be arranged in a row so that adjacent balls are of diferent colours, is