The first way to calculate the expected uses the marginal percentages. If sex is not related to flavor preference, you would expect the same percentages of males to prefer the same flavors as females. Since overall 20.71% prefer vanilla, you would expect 20.71% of males to prefer vanilla. Now, 20.71% of 84 people is 17.3964, so this is how many males you would expect to prefer vanilla. You would also expect 20.71% of the females to prefer vanilla, which is 17.6035. So you'd expect 17.3964 males and 17.6035 females to prefer vanilla.
