6. The reading on a factory's electricity meter is 52222 kilowatts. After several days, the meter reading (a five-digit number) again shows four identical digits. How much electricity, in kilowatts, did the factory use at least in these days?

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【Analysis】According to the problem, we only need to find the number with four identical digits that has the smallest difference with 52222. Starting from 52222, the number with four identical digits is 52555, and it is not difficult to find the number of kilowatt-hours used.

【Solution】According to the analysis, if there are four identical digits, then starting from the units place, when the last three digits are the same and the same as the ten-thousands place 5,

at this point, the number is the smallest. Starting from 52222, the number with four identical digits is 52555, and the electricity used is: $52555-52222$ $=333$ (kilowatt-hours).

Therefore, the answer is: 333.