Read each sentence Rewrite it correctly using semicolons in place of commas to avoid confusion

The assignment is Chapter 20, pages 420-425, all of Chapter 21, and Chapter 22, pages 506-515
The assignment is Chapter 20, pages 420-425; all of Chapter 2, and Chapter 22, pages 506-515

Classes will be in the gym on Tuesday, October 5, Tuesday, October 12, and Tuesday, October 19.
Classes will be in the gym on Tuesday, October 5; Tuesday, October 12; and Tuesday, October 19

The baby, Lily, the twins, Kevin and Kayla, and Mom and Dad are at the pool.
The baby, Lily; the twins, Kevin and Kayla; and Mom and Dad are at the pool

THe store sells clothing, children's teens', dolls, games, board and video, and books.
The store sells clothing; children's and teens, dolls, games;board and video, and books.

I disagree with your last answer. The others are correct.

Where do I put the semicolon and where do I replace the commas? I'm confused on this one

In my opinion, this exercise is stretching the use of a semicolon. You could probably write a book without using that mark of punctuation.

The store sells clothing, children's and teens; dolls; games,board and video; and books.

The assignment is Chapter 20, pages 420-425; all of Chapter 21; and Chapter 22, pages 506-515.

Classes will be in the gym on Tuesday, October 5; Tuesday, October 12; and Tuesday, October 19.

The baby, Lily; the twins, Kevin and Kayla; and Mom and Dad are at the pool.

The store sells clothing; children's and teens'; dolls, games; board and video; and books.