Optimization question about minimizing cost

a new cottage is built across the river and 300m downstream from the nearest telephone relay station. the river is 120m wide. in order to wire the cottage for phone service, wire will be laid across the river under water and along the edge of the river above ground. the cost to lay wire under water is $15 per m and cost to lay wire above ground is $10 per m. How much wire should be laid underwater to minimize the cost?

Here is what I have:


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in the end I get x=3 but I don't think that is right.




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This should be mathematics. Your opinion is not particularly important. Prove it! Check you work.

After "=0", how did you manage "=10"? Very bad. 0/10 = 0

Thank you for displaying your work.

Your welcome and it should be 0/10? why not 0+10 for this reason i put 10. :S

Solve for x:

10*x = 0

10 + x = 0

You must see this error or you need to take a week off and work on liberal arts, or something, then come back to it.