Sharon called 3 friends on Friday. On Saturday, each of those 3 friends called 3 different friends. On Sunday, each person who was called on Saturday called 3 different people.

How many friends were called on Sunday?

3 were called on Friday.

Then the each of the 3 called 3 others which all equals to 9, since 3*3 is 9.

Now, each of the 9 called 3 others on Sunday which altogether equals 27, since 9*3 is 27. Therefore, the answer is 27.

Every time the word each is used in a math problem, that would mean that you need to multiply something. And that's what happened here