The secret number is the least whole number that has three different prime factors. Since, this is the least whole number it should have the 3 smallest prime numbers as its factors.
The 3 smallest prime numbers are: 2, 3 and 5
The number that will have 2, 3 and 5 as its only factors will be the least whole number with 3 different prime factors.
, you have to factor it as 
, not 
, because that will leave an extra 6x because of 8x - 2x. 