If Mrs. Smith uses 5 lemons to make 2/3 gallon of lemonade, we can set up a proportion to find out how many gallons she can make with 15 lemons:
5 lemons / (2/3 gallon) = 15 lemons / x gallons
To solve for x, we can cross-multiply:
5 lemons * x gallons = (2/3 gallon) * 15 lemons
5x = 10
x = 2
Therefore, Mrs. Smith can make 2 gallons of lemonade with 15 lemons.