Janet buys 45-pound containers of cat litter for $21 each.  If her cat litter box holds 15 pounds of cat litter, and she changes out the litter weekly, how much will it cost, in dollars, for her to buy enough litter to last 210 days?
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