Answer: She can buy 47 dog biscuits.
To solve this equation we can simply subtract $1.80 from $30 and get $28.20. With the remaining $28.20 you can divided by $0.60 to get how many total dog biscuits your sister can buy. 28.20 divided by 0.60 is 47.
So your sister can buy 47 dog biscuits.
