If she changes the box weekly, then in 210 days, she will change the box 210/7=<<210/7=30>>30 times.
At 15 pounds of litter for every box change, she will require 30*15=<<30*15=450>>450 pounds of cat litter in 210 days.
If she buys 45-pound containers, then for 450 pounds she will need to buy 450/45=<<450/45=10>>10 containers of litter.
At $21 per container, to buy enough cat litter for 210 days, it will cost 10 * $21 = $210
#### 210