Jack really likes his number five: the trick here is that you have to multiply each number by 5 raised to the number of digits of each number, so, for example:
Kata.multiply(3) == 15 // 3 * 5¹
Kata.multiply(10) == 250 // 10 * 5²
Kata.multiply(200) == 25000 // 200 * 5³
Kata.multiply(0) == 0 // 0 * 5¹
Kata.multiply(-3) == -15 // -3 * 5¹