What is a cent in singing?
A cent is a hundredth of a semitone. Two neighbouring piano keys are 100 cents apart.
When we say you sang 30 cents flat, the note was about a third of the way down to the next key. We call a note in tune within ±25 cents, because voices naturally waver and vibrato swings wider than that.
More questions: Is there an app that checks if I'm singing in tune? · How accurate is an online pitch or vocal range test? · Can you learn to sing in tune? · How do I find my vocal range? · How do I sing high notes without straining?