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Method 1
(3 drinks)/(9 dollars) = (5 drinks)/(x dollars)
3/9 = 5/x
3x = 9*5 ... cross multiply
3x = 45
x = 45/3
x = 15 dollars
Method 2
3 drinks cost $9, which means the unit price is 9/3 = 3 dollars per drink.
Five such drinks gets a total price of 5*3 = 15 dollars