Answer:
1000 seconds
Step-by-step explanation:
You can start by finding the unit rate for each of them to lay down a tile. That is, how many tiles they can lay down in one second. Ralph can lay one down in 1/5 of a second, while Bob takes 1/4. 1/5+1/4=4/20+5/20=9/20 of a tile per second. Dividing 450 by 9/20, you get 1000 seconds. Hope this helps!
