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*assuming that ABC is a straight line**
<span>then you have </span>
<span>AB = 17.89 ..... this is the length of the whole line </span>
<span>BC is a part of the line = 7 </span>
<span>therefore the length of the other part = 17.89 - 7 </span>
<span>= 10.89</span>
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.