Suppose: when the amount of "peak electricity" is X, it is more economical to use "peak electricity".

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".

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