For part A Amy's estimate is inaccurate because when adding fractions they must have the same denominators. Amy probably made the mistake of adding 1/4 and 1/3 without changing the denominators first which gave her 1/7. If she were to do it correctly then she would've ended up with 7/12 as her answer.
My work:
1/4=3/12
1/3=4/12
4/12+3/12=7/12
I hope this helps and if I gave any false information I apologize in advance.