physics aptitude question

Q: A baby's bath should be at a temperature of 37'C. The bath alreadycontains 10kg of water at 15'C. Approximately how much hot water, with atemperature of 50'C, should be added to achieve the desired temperature?

A: 17kg

I'm not sure why it's 17kg. I know it's an easy question but it eludes me.

Btw I put this on the maths forum because it's more active + mathsy people tend to be just as adept at physics

Ummmm, this isn't a particularly physicsy question if I've got the right idea. The way I would do it:

the 15'C water is 22'C too cold, whereas the 50'C water is 13'C too hot. If you find the ratio of too cold to too hot, it's:

22:13 which is 1.69(ish):1

Therefore, if we want to balance this temperature disparity, the weight of water should intuitively be in the opposite ratio, or:

1:1.69(ish)

We know we have 10kg of the too cold water, so we want 1.69x10=16.9kg of the too hot water, which is approx 17kg Hope that helps

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