>Thanks Hilmar! Yes you're correct that my answer is purely a quick assumption. Where is that puzzle section anyway?

Chatter | Puzzles. But I am not sure whether it is appropriate in this case. Well, I mentioned it because this kind of problem might typically appear in puzzle sections.

It is interesting to note that the first guess I did - many years ago - for this problem was exactly your approach. I think I later learned the correct approach from algebra books or something.

Hilmar.
>
>>>If using hose A, I can fill a drum in 20 minutes but if I am going to use hose B, I will only take 18 minutes. If I will use both (A & B), how many minutes will I spend to fill the drum?
>>>
>>>30 + 18 = 38; 38/2 = 19, 19/2 = 9.5 which is my answer. Is this correct?
>>
>>You could have placed this in the "Puzzle" section < G >.
>>
>>The exact calculation goes like this:
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>>With hose A, every minute you fill 1/20 of the container.
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>>With hose B, every minute you fill 1/18 of the container.
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>>With both hoses, every minute you fill 1/20 + 1/18 of the container, or 10/180 + 9/180 = 19/180.
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>>It takes 1 / (19/180) minutes to fill the container with both hoses, that is, 180/19 minutes. That is 9 + 9/19 minutes, or 9.4737 minutes.
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>>Your calculation is incorrect, but close, and it can be used as a quick estimate. However, if the difference between the two times is much greater, the discrepancy between the approximate and the exact answer will be greater.
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>>For instance, if it takes 1 minutes with hose A, and 10 minutes with hose B, your calculation would give 5.5 minutes, which is way off (it should be obvious that with both hoses, the time taken must be less than the time for each individual hose).
>>
>>Now, the review question: Jupiter takes 12 years to go around the sun, Saturn, 30. If at this moment Jupiter and Saturn are aligned, when will they be aligned again? (This requires subtraction instead of addition. The answer is 20 year, seen from the sun; from the Earth, the situation gets complicated by the Earth's movement.)
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>>Hilmar.