Liquid helium boils at 4.2K. What is the boiling point of helium in degrees Celsius?

Question

I understand that you subtract 273.15. Then you get -268.95, but my book says the answer is -270. How do sig figs and rounding make that -270?

Answer 1

Your textbook is wrong. Note though that 4.2 is not as precise as 273.15, so you need to round your answer to -269.0; this is the answer you should write.

Answer 2

T(K)=273.15+T(Celsius),  4.2=273.15+T(celsius) ,

T(celsius)=-268.95  which is approximately -270 !