Seven

Seven dwarfs will cut 420 stumps in 15 hours. After five hours, the two dwarves disappear discreetly. How many hours will the remaining dwarves complete the task?

Result

x =  14 h

Solution:

t=420/(7 15)=4   420=5 t 7+(72) x t  420=5 4 7+(72) x 4  20x=280  x=14 =14 ht=420/(7 \cdot \ 15)=4 \ \\ \ \\ \ \\ 420=5 \cdot \ t \cdot \ 7+(7-2) \cdot \ x \cdot \ t \ \\ \ \\ 420=5 \cdot \ 4 \cdot \ 7+(7-2) \cdot \ x \cdot \ 4 \ \\ \ \\ 20x=280 \ \\ \ \\ x=14 \ \\ =14 \ \text{h}



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