Kevin will water both of his plants on the 15th.
The lowest common multiple of 3 and 5 is 15. Therefore, he will water both then.
Here ... I'll give you all the help you need to answer this question
on your own:
-- The three angles inside any triangle always add up to exactly 180 degrees.
-- Simply examine each group of numbers in the question.
Any set that adds up to 180 can be the three angles of a triangle.
Any set that doesn't, can't.
Go to it !