1、( 1) 10! /8! = 10×9=90

(2) 100! /99! = 100

2、[780×85%-780÷( 1+30%)]× 12

=[663-600]× 12

=756 yuan

So what? The profit for this day is 756 yuan.

3. Set the peak time to x hours and the trough time to 100-x hours.

Then (2/3)x= 100-x

Solve? x=60

The peak time of the family is 60 hours, and the low time is 40 hours.

According to the time-sharing pricing table, the electricity fee is 60×0.56+40×0.28=44.8 yuan.

Electricity charge without time-sharing: 100×0.53=53 yuan.

∴ Installing time-sharing meters can save 53-44.8=8.2 yuan.

5. As shown in the figure below:

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