In Java 15

public static String kangaroo(int x1, int v1, int x2, int v2) {

 //x1 + v1*f = x2 + v2*f
 //(x1-x2)+(v1-v2)*f = 0

    String result = "";
    double f = (double) (x2-x1)/(v1-v2);

    if(f < 0 || (f*10)%10 != 0){
       result = "NO"; 
    } else {
        result = "YES";
    }
    return result;
}