For beginners: using this method:
X be the number of 20 Ltr bottles and Y be the number of 40 Ltr bottles
(20X+40Y)/48=35
20X+40Y= 48*35
Let 'a' be the number of bottles which needs to be removed to get the new average of 25
The new average equation is (20X+40(Y-A))/(48-A)=25
20X+40Y-40A=25*48-25A (Substitute 20X+40Y as 48*35 from above equation)
From this A=32
dred
A convenience store currently stocks 48 bottles of mineral water. The bottles have two sizes of either 20 or 40 ounces each. The average volume per bottle the store currently has in stock is 35 ounces. How many 40 ounce bottles must be sold for the average volume per bottle to be reduced to 25 ounces if no 20 ounce bottles are sold?
(A) 10
(B) 20
(C) 30
(D) 32
(E) 34