SOLUTION: The country's new paving machine can surface 1km of highway in 8 hours, whereas it takes the old one 12 hours. How long will it take them to surface 20 km of highway if they start

Algebra ->  Rate-of-work-word-problems -> SOLUTION: The country's new paving machine can surface 1km of highway in 8 hours, whereas it takes the old one 12 hours. How long will it take them to surface 20 km of highway if they start       Log On


   

Question 544460: The country's new paving machine can surface 1km of highway in 8 hours, whereas it takes the old one 12 hours. How long will it take them to surface 20 km of highway if they start at opposite ends and work day and night?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
the new one's rate of paving is +1%2F8+ km/hr
The old machine's rate is +1%2F12+ km/hr
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Add their rate off paving to get their rate of
paving together, +20%2Ft+, where t is
their time in hours to pave 20 km
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+1%2F8+%2B+1%2F12+=+20%2Ft+
Multiply both sides by +24t+
+3t+%2B+2t+=+20%2A24+
+5t+=+480+
+t+=+96+
It will take them 96 hrs