Problem with higher numbers – Fraction Math Review

Trouble is, if one wants to type in something like 10 over 7 on one side and 15 over 7 on the other side, the numbers appear as -1..., whatever it is. In other words, a bug. So I could only give this program ****. Haven't tried the multipli. or division sections yet because I just don't feel ready for them, so am basing this review on the addition and subtraction parts.
Review by Beth Kaplan on Fraction Math.