50. To perform large-scale computations using computers, people currently generally adopt the following two methods:

The first traditional method is to build a supercomputer. This method was widely used in the past, but people gradually found that building a single supercomputer is not cost-effective, as its computing power is proportional to the square root of the cost.

The other more recent technology is to build a distributed computing system. It involves using a large number of low-performance computers (also called workstations) to form a computing network. Such a network has astonishing computing power because the total computing power of the network is the sum of the performance of each workstation.

Suppose the unit of computing power of a computer is MIPS (i.e., the number of millions of instructions executed per second), a traditional supercomputer with a computing power of 6000 MIPS costs 1 million yuan; in a distributed system, each workstation has a computing power of 300 MIPS, and its price is only 50,000 yuan. It should be noted that building a distributed computing system requires a high level of technical expertise, with initial R&D and network construction costs amounting to 600,000 yuan.

Question: At what level of investment is it more cost-effective to build a new distributed computing system?
50. Let the invested capital be $x$ million yuan, and the computing power achieved by the two methods be $y_{1}, y_{2}$ MIPS,

then $y_{1}=k_{1} \sqrt{x}$, substituting $x=100, y_{1}=6000$ into the above equation gives $k_{1}=600$,

and $y_{2}=k_{2}(x-600)$, when $x-600=5$, $y_{2}=300$ substituting into the above equation gives $k_{2}=60$,

from $y_{2} \geqslant y_{1}$ we get $60(x-600) \geqslant 600 \sqrt{x}$, i.e., $x-10 \sqrt{x}$ $-600 \geqslant 0$. Solving this gives $x \geqslant 900$ million yuan.