We don't know if the Kardashians make this easy or hard to figure out but don't despair if you find out that the answer was so easy, because we all have our bad days and good days. We all miss small details while in the rush of our lives, so it's all ok. Here is the solution process...
We have 3 Kims that add up to 30. So 30 divided by 3 is 10. So each Kim Kardashian is 10.
We have one Kim, who is 10, and 2 Kendall heads, which add up to 18. And 10 + 8 = 18. Both Kendalls are 8, so Each Kendall is 4.
We have a Kendall minus Kylie. Kendall is 4, and (4 - Kylie =2), so Kylie is also 2.
Kim is 10, Kendall is 4 and Kylie is 2, but X comes first, so 4X2 =8 and then 8 + Kim... 8+10 = 18
So the correct answer is 18.