> How do you ever decide whether a load of laundry needs 30 or 40? Do you sum up all the laundry labels on each item of clothing then take the average?

If you have enough laundry I expect you try to put stuff with similar requirements together in the same way you segregate whites and colors.

Though usually unless your laundry is quite dirty you'd go with 30 standard and shove everything in there, at 30 color shouldn't even be too much of a factor.

In all honesty I'm not quite sure why both are present, I think it's because older generations believed 30 would not be enough (as it's "human range" water), so 30 or cold would only be for the clothes which can't go any higher and 40 was the baseline.