Let’s see, for 6 servings he needed 4 carrots, meaning every carrot was equal to 1.5 servings. 15/1.5 is 10, 10 carrots for 15 servings.
First, 4 people know the post
Then, those 4 people share it each with 4 people (4 * 4 = 16)
Then, each of those people share it with another 4 people (16 * 4 = 64)
And so on. the function is as simple as: 4^x (4 to the power of x)