NobodyAnyone
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- Dec 13, 2021
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I just want to know why isn't this correct and what formula should I use for this. I used cross product for this problem.
Simple math problem that I couldn't figure out
- Thread starter NobodyAnyone
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I just want to know why isn't this correct and what formula should I use for this. I used cross product for this problem.
Steven G
Elite Member
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1st, never use cross multiplication--never!
25/8 = x/40. Just multiply both sides by 40. Why would you multiply by 8, just to then divide by 8. I understand that was what you were taught but it is completely inefficient.
Also, why would the units be in feet?
25/8 = x/40. Just multiply both sides by 40. Why would you multiply by 8, just to then divide by 8. I understand that was what you were taught but it is completely inefficient.
Also, why would the units be in feet?
NobodyAnybody, for each part, think number (of percent) multiplied by the quantity.
The total quantity is 50 ml.
If you let the needed unknown ml. of 20 percent acid be x, then that forces the number
of ml. of pure acid, which is 100 percent strength, to be (50 - x).
Solve this equation (try to come up with it first before clicking the spoiler):
By the way, with the answer choice I see, you are dealing with a rounded value. Whether that
has to do with significant digits of the values in the problem, I will leave to others to address.
The total quantity is 50 ml.
If you let the needed unknown ml. of 20 percent acid be x, then that forces the number
of ml. of pure acid, which is 100 percent strength, to be (50 - x).
Solve this equation (try to come up with it first before clicking the spoiler):
\(\displaystyle 20x \ + \ 100(50 - x) \ = \ 45(50) \)
By the way, with the answer choice I see, you are dealing with a rounded value. Whether that
has to do with significant digits of the values in the problem, I will leave to others to address.
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