Answer:
5
Step-by-step explanation:
Since, A, B, C, D are collinear.
[tex] \implies A - B - C - D \\
\therefore \: AD = (AB + BC) + CD \\ \therefore \: 18 = AC + CD.. (\because AB + BC =AC) \\ \therefore \: 18 = 8 + CD \\ \therefore \: 18 - 8 = CD \\ \therefore \: 10 = CD \\ \\ \because \: BC + CD = BD \\ BC = BD - CD \\ BC = 15 - 10 \\ BC = 5 \\ [/tex]
