Answer:
Obtuse 
Step-by-step explanation: Plugging the numbers into the Pythagorean Theorem (a^2 + b^2 = c ^2) 
20^2 = 12^2 + 15^2 
400 > 144 + 225
400 > 369 
Because of the Converse of the Pythagorean Theorem, which tells us if c is greater than a+b, it is obtuse, we know the answer to be obtuse. I hope this makes since!!