An offshore oil rig is located in the ocean at a point A, which is 10 mi from the closest point B on the shore, on a straight shoreline. The oil is to be piped to a point D on the shore that is 50 miles away from point B. The cost of laying pipe is $2 million per mile under water and $1.5 million per mile over land. The most cost-effective way of constructing the pipeline would be to connect the oil rig to a point C on the shore between B and D (i.e. segment AC is completely underwater) and then build the remaining segment between over land between C and D. If point C is x miles away from point B, find the length of x that will minimize construction cost for the entire pipeline.