Russell ate several candies on Halloween, and each day after Halloween ate 6 fewer candies than the day before. His candies lasted exactly five days, and he ate 100 candies in total. How many did he eat on the third day?

Let a = the number of candies eaten on the first day.

Because the number of candies eaten decreases by 6 every day after, the number of candies eaten creates the arithmetic sequence

a, a-6, a-12, a-18, ...,

After 5 days, the total candies eaten is 5a - 60.

Therefore

5a - 60 = 100

5a = 160

a = 32

On the third day, the number of candies eaten is

a - 12 = 32 - 12 = 20

Answer: 20