For a single 20 sided dice, a chance to get a certain number would be 1/20. For 2 numbers it will be a 2/20 chance, which can be simplified to a 1/10 chance that you will roll a 15 or a 17.
Answer:
The percent decrease is approximately 
(Don't know what you are expected to round to.)
Step-by-step explanation:
Percent change can be calculated by doing:

Then you multiply the result to covert it to a percentage.
Let's use this formula:




approximately
Now the answer as a percent is
approximately.
The percent decrease is approximately 
18.03 inches
Put the dimensions into the pythagorean theorem, that will give you the answer.