How to round numbers when performing linear interpolation?

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Say I have to find the number of people above a certain mass. I can do linear interpolation to get a value that will give me the number of people below this mass and I can simply subtract this from the total number of people to get my answer. However, I'm a little confused regarding decimal numbers. If 23.8 people out of 31 are below 63 kg, how do I get logically (rounded) accurate values for the number of people below and above 63 kg?