I really enjoyed spending a bit of my morning with these two implementations of the concept.

I prefer your idea of treating the question as a proper puzzle with a 1-submission limit. The "calculator" UI took a whole 2 minutes to understand initially, but I really liked seeing the chain instead of having to mush all the factors in my head.

It's really nice to see the correct answer broken down to get a feel about the real numbers!

The current question's answer seems to contain big errors in magnitude in its factors:

"How many kilograms of skin does a human shed in their lifetime?"

    Skin cells shed per day: 5e8 / day
    Mass of one skin cell: 3e-6 g
    Years in a lifetime: 80 yr
    Grams to kilograms conversion: 1 kg / 1000 g

The first two factors correspond to shedding 1.5 kg/day, which is definitely unrealistic!