Depends how accurate you want your measurement:
The number looks like the result of a calculation, so to simplify it, round it to an appropriate number of significant figures: the number being dependant upon what you are measuring and how you measured it - ie what kind of accuracy you used to measure whatever it was in the first place.
reasonable simplifications would be:
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It is 16/20 which can be simplified, if required.
144 inches : 2.5 feet = 144 : 30 = 24 : 5
0.31 inches = 31/100 inches and that fraction cannot be simplified further.
0.321 inches = 321/1000 inches. The GCD of these numbers is 1, so the fraction can't be simplified any more.
The ratio of 153 inches to 17 feet is 153:204, or simplified as 3:4.