break these questions down and explain why the answers are these

19. A vecurity box guarding a scasitive area bas throe buttons colored red, green, and blud. To enter the area, you must enfer a sectirity code, pressing four buttons in succession. How many different security codes are there? R G B 3333=81 20. Five poople are chosen from a group of 20 to take a trip. In bow many ways can this be done if A and B refuse to travel without each other? (Note, it may be that neither one travels) ABdonottravel+ABtraveltogetherORTotal-Atravels,notB-BtravelsnotAC(18,5)+C(18,3)+C(20,5)C(18,4)=9384C(20,5)C(18,3) 21. In how many ways can 7 identical Art books and 13 identical Math boeks be ditubuted anoug 4 student? 22. How many ways can four funilics (with three persons in eacti family) stand in a movie thesat line if each family must say together? 3!3!3!3!4:31,104