0.56 * x+0.28 *( 140-x)& lt; 0.53* 140
0.56 * x+0.28 * 140-0.28 * x & lt; 74.2
0.28 * x+39.2 & lt; 74.2
0.28 * x & lt35
x & lt 125
Answer: When the amount of "peak electricity" is less than 125, it is more economical to use "peak electricity".
You can ask questions; Help, please adopt. I wish you progress in your study.