SOLUTION: it takes robert and william 12 hours to complete a history project. If Robert works alone on the same project for 16 hours and let william take over, william needs another 7 hours

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Question 1087237: it takes robert and william 12 hours to complete a history project. If Robert works alone on the same project for 16 hours and let william take over, william needs another 7 hours to complete it. How long does it take william to complete the history project alone?
Answer by josgarithmetic(39620) About Me  (Show Source):
You can put this solution on YOUR website!
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...robert and william 12 hours to complete a history project. If Robert works alone on the same project for 16 hours and let william take over, william needs another 7 hours to complete it.
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system%281%2Fr%2B1%2Fw=1%2F12%2C%281%2Fr%2916%2B%281%2Fw%297=1%29



BETTER APPROACH:
If use rates just as variables, not in a form of ratioinal numbers with times as denominators,...
system%28R%2BW=1%2F12%2C16R%2B7W=1%29
-
R=1%2F12-W
16%281%2F12-W%29%2B7W=1
16%2F12-16W%2B7W=1
4%2F3-9W=1
4%2F3-3%2F3=9W
1%2F3=9W
highlight%28W=1%2F27%29

William could do the whole job alone in 27 hours.