Why in Math.random() multiplied by 5? why not other numbers?

Hello everyone i'm beginner and now i'm at polymorphism section, and i found something hard to understand, and this is the first time i saw Math.random(), The question is why the instructor use *5, why not *6 or *4 or other numbers?, the second question is why he used +1 especially? Sorry for my bad english This is the source code that i'm talking about:

package CompositionChallenge;

class Movie {
private String name;

public Movie(String name) {
    this.name = name;

}

public String plot() {

    return "There is no plot";

}

public String getName() {
    return name;

} }

class hacksawRidge extends Movie {

public hacksawRidge() {
    super("Hacksaw Ridge");

}

@Override
public String plot() {

    return "Talk about world war 2 and how a medical person helps people to live their life";

}}

class Love extends Movie {

public Love() {

    super("Love");

}

@Override
public String plot() {

    return "talk about a person has sex every single day with his wife and girlfriend";

}}

class TripleThreat extends Movie {

public TripleThreat() {

    super("TripleThreat");

}

@Override
public String plot() {

    return "The best fighters in the world in one fucking movie";

}}

public class Main {

public static void main(String[] args) {

    for (int i = 0; i <= 4; i++) {
        Movie movie = randomMovie();
        System.out.println(
                "Number " + i + " and the random movie is " + movie.getName() + " and its plot is " + movie.plot());

    }}

public static Movie randomMovie() {

    int randomNumber = (int) (Math.random() * 5) + 1;

    System.out.println("Random number generated was " + randomNumber);
    switch (randomNumber) {
    case 1:
        return new Love();
    case 2:
        return new hacksawRidge();

    case 3:
        return new Movie("No movie now");
    case 4:
        return new TripleThreat();
    default:
        return null;

    }
}}