Lennie,
Thank you for your question.
I agree with Violet227; mathematically “15 over 3” and “fifteen thirds” have the same value.
If you are transcribing the print you attached (thank you for including the example), you could use either the simple horizontal line (a series of dots 25) or the spatial fraction line (a series of dots 25 preceded by the opening fraction indicator and followed by the closing fraction indicator) between the numerator “15” and the denominator “3”.
–Kyle