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An easy way to convert lbs -> kg is divide by 2 and subtract 10%

Just divide by 2. No need for the 10% bit.

10% is small enough that it does not matter for these types of informal conversational scenarios, and in cases where it does, do a proper conversion.

3000lbs to KG Mental Math

/2 = 1500 .1 = 150 = 1500 - 150 = 1350kg

Google's result: 1360.777kg