calculator continually gives wrong calculation
I was recently using my iPhone 5S to calculate grades for students on a test. Overall total points were 120 and I would subtract 42 points from 120 and my calculator would give me 77 each time. I cannot understand why it gave me this answer 3 times in a row. Anyone else having the same problem? Also how can I contact Apple so they know their calculator isn't functioning correctly?
iPhone 5s, iOS 8.1.2