Answer:
Amy and her sister will have 17.5 gallons of lemonade for the school dance.
Explanation:
The sister's recipe makes 2 quarts of lemonade. As they want to make 35 times more, so total number of quarts made will be = 
 
 quarts 
 
Now, 4 quarts has 1 gallon. 
 
So, total number of gallons in 70 quarts will be = 
 
tex]\frac{70}{4} =17.5[/tex] gallons. 
 
Hence, the answer is 17.5 gallons.