s = s_{1} + s_{3} - s_{2} = 3 k m + 12, 5 k m - 5 k m = 10, 5 k m

${s = s_1 + s_3 - s_2 = 3 \,\mathrm{km}+12,5\,\mathrm{km} - 5\,\mathrm{km} = 10,5\,\mathrm{km}}$

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