Answer: 7 hours.
Step-by-step explanation: Randall can assemble 5 race car tracks in an hour and Diggery can assemble 4 race car tracks in an hour. To see how many hours it takes to make 63 race car tracks, you need to add 5 and 4, since Randall and Diggery are working together. Both of them combined can make 9 race car tracks in total every hour. 63 can be divided by 9, which is 7. Therefore, it'll take 7 hours for both of them to make 63 race car tracks.
Let me know if this is right! :)