The relationship between atmospheres and mmHg is ##”1atm” = “760mmHg”##

You can take that equality and come up with two conversion factors, or ratios, equal to 1:

##”1atm”/”760mmHg”## = ##”760mmHg”/”1atm”## = ##”1″/”1″##

Using dimensional analysis, multiply 125mmHg times the conversion factor that has the atm unit on top and mmHg on the bottom, so that mmHg will cancel, leaving atm in the answer.

##”125mmHg”## x ##”1atm”/”760mmHg”## = ##”0.164atm”## The mmHg would cancel, leaving you with atmospheres in your answer.