OG详解-OG5 数学2 Q20

The correct answer is 20. Adding the first equation to the second equation in the given system yields 5y - 5y = 10x + 5x + 11-21, or 0 = 15x-10. Adding 10 to
both sides of this equation yields 10 = 15x. Multiplying both sides of this equation by 2 yields 20 = 30x. Therefore, the value of 30x is 20.