Explanation:
If Sally has 2 cats that each eat 1/4 of a tin of food each day, that means that the cats are eating 1/2 tin each day.
If the cats eat 1/2 each day, that means 1 can will last the cats 2 days.
If Sally buys 9 cans, that means she will cat food for 18 days.
1/4 + 1/4 = 1/2
1/2 per day = 1 tin every 2 days
9 tins x 2 days = 18 total days of food