Answer: A decrease to 30 minutes from 3 hours corresponds to a 83.3% relative decrease, or a reduction to a time period that is 16.7% of the original period (depending on a more precise meaning of your question, see below)
Steps:
Relative decrease from y to x:
100*(y-x)/y in %
So y = 3 hr and x = 30 min. Rewrite to fractions: x=6/2 hr, y=1/2 hr
rel decrease =
Percentage of reduced time period:
the 30 minutes time period corresponds to of the full period of 3hr, or 16.67% (this is in case your question was asking for the percentage the remaining amount represents of the total amount).
Let me know if you have questions.