The cost, in millions of dollars, for a company to manufacture x thousand speed boats is given by the function C(x) = 3x2 - 30x + 225.

The cost, in millions of dollars, for a company to manufacture x thousand speed boats is given by the function

C(x) = 3x2 - 30x + 225.

Find the number of speedboats that must be produced to minimize the cost.

To minimize the cost, the company must produce ____  thousand speedboats.