SOLUTION: Please help me: A boy walked over hills at 3 kilometers per hour and on flat roads at 4 kilometers per hour. He walked 18 kilometers in 5 hours. How much of his trip was over hill

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Question 139700: Please help me:
A boy walked over hills at 3 kilometers per hour and on flat roads at 4 kilometers per hour. He walked 18 kilometers in 5 hours. How much of his trip was over hills?
Thanks
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Let h= km walked on hills
Let f= km walked on flat groun
h+%2B+f+=+18
h%2F3+%2B+f%2F4+=+5
-------------------
f+=+18+-+h
Use substitution
h%2F3+%2B+%2818+-+h%29%2F4+=+5
Multiply each side by 12
4h+%2B+3%2A%2818+-+h%29+=+60
4h+%2B+54+-+3h+=+60
h+=+6km
f+=+18+-+h
f+=+18+-+6
f+=+12km
6 km of the trip was over hills
check answer:
h%2F3+%2B+f%2F4+=+5
6%2F3+%2B+12%2F4+=+5
2+%2B+3+=+5
OK