How many liters of pure alcohol must be added to 15 liters of a 20% solution to obtain a mixture which is 30%?

Question

Pls. help me answe this...thank u =)

Answer 1

First, figure out how much alcohol and how much water (in liters) is in there now. (What's 20% of 15l? The rest is water.)

Look at the amount of water. This is going to be 70% of your final amount. If the amount of water is 70%, how many liters are 100%?

And how many liters are 30%?

Substract the amount of alcohol already in the solution from that number. The difference is the amount of alcohol you have to add.

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Edited to add: By the way, I'm sorry, but Checkmate's answer (below mine) is incorrect. If you add 1.5l of alcohol, you no longer have 15l, you have 16.5l. So 4.5l would no longer be 30% - it'd only be 27.3%. Sorry.

Answer 2

Assuming that 15 liters of a 20% solution means that 20% of the 15 liters are alcohol, we can figure out that 3 liters of the 15 liters of the 20% solution are alcohol, because 15 x 0.20 = 3

However, the solution needs to be 30%. We know that 30% of 15 liters is 4.5 liters of alcohol, because 15 x 0.30 = 4.5

Since there are 3 liters of alcohol in our 20% solution, we only have to add 1.5 liters of alcohol ( 3 + 1.5 = 4.5 ) to obtain the 4.5 liters needed for a 30% mixture of 15 liters.

I repeat the final answer just to make sure you got it, 1.5 liters of alcohol must be added.

Answer 3

Homework, huh?