Question 1193949
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Don't pay a lot of attention to tutor @ikleyn's "standard mantra" for how to solve this kind of problem.  There are numerous ways to solve them; what is her favorite ("standard") way of solving them isn't going the be everybody's favorite method.<br>
I would simply use common sense to solve this problem.  If the "new" number of girls is 3/7 of the original number, then the amount of time required to do the work is 7/3 as much.<br>
ANSWER: (7/3) of 1 hour, or 7/3 hours, or 2 1/3 hours, or 2 hours 20 minutes.<br>
If you want to be just a bit more formal with this same solution path, you can say that the number of hours required to do the work is inversely proportional to the number of workers (more workers means less time; fewer workers means more time), so 3/7 as many workers means (1/(3/7)) = 7/3 times as many hours.<br>
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In response to the note from tutor @ikleyn about my "attacking" her....<br>
Yes, tutor @ikleyn, I will ABSOLUTELY attack you any time you tell a student that your way of solving a problem is THE way.<br>
This kind of problem is very easily solved using common sense.  If you don't have common sense and want to solve the problem using the concept of "girl-hours", then that's fine.  But don't tell students that is the only way to solve the problem.<br>
And don't "attack" me for suggesting that the student solve the problem using logical reasoning instead of formal mathematics.<br>