Patterns & Reasoning

Problem

Subtract the overlap when joining fraction lengths

Five points A, B, C, D, E sit on a line in that order. I'm given the spans AC, BD, DE, and the overlapping span BC. Using the number-line figure I must find the total distance from A to E.
A B C D E 17 2/5 km 19 4/5 km 4 3/5 km 6 1/5 km
Fractions
Your answer
km
How to solve
Strategy Draw a Diagram — The figure shows A–B–C–D–E with AC and BD sharing the overlap BC. Adding AC + BD double-counts BC, so AE = AC + BD + DE - BC. The diagram makes the overlap easy to subtract.
1STEP 1

Read the figure and set up the spans

AC and BD overlap on BC, so adding them double-counts it: AE = AC + BD + DE - BC.

AE = AC + BD + DE - BC
2STEP 2

Add the whole-number parts

Combine the whole numbers: 17 + 19 + 4 - 6 = 34.

17+19+4-6=34
3STEP 3

Add the fraction parts

All fractions share denominator 5: 25\frac{2}{5} + 45\frac{4}{5} + 35\frac{3}{5} - 15\frac{1}{5} = 2+4+315\frac{2+4+3-1}{5} = 85\frac{8}{5} = 1 35\frac{3}{5}.

25+45+3515=85=135\frac{2}{5}+\frac{4}{5}+\frac{3}{5}-\frac{1}{5}=\frac{8}{5}=1 \frac{3}{5}
4STEP 4

Combine for AE

Add the whole-part total and the fraction-part total: 34 + 1 35\frac{3}{5} = 35 35\frac{3}{5}.

34+135=353534+1 \frac{3}{5}=35 \frac{3}{5}
Answer
35 35\frac{3}{5} km
34 + 1 35\frac{3}{5} = 35 35\frac{3}{5}
Cross-check with segment pieces: AB = AC - BC = 11 15\frac{1}{5}, CD = BD - BC = 13 35\frac{3}{5}, so AE = AB + BC + CD + DE = 11 15\frac{1}{5} + 6 15\frac{1}{5} + 13 35\frac{3}{5} + 4 35\frac{3}{5} = 35 35\frac{3}{5} km. Same answer, and 35 35\frac{3}{5} is sensibly larger than each given span.
Takeaway

This only needs Grade 4 fraction add/subtract — draw the line and remember the overlap gets counted twice, so subtract it once!

  • Read the figure and set up the spans
  • Add the whole-number parts
  • Add the fraction parts
  • Combine for AE
Where next?
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