Calculate BC.

In ∆ABD,

$\text{tan 35°} = \dfrac{\text{Perpendicular}}{\text{Base}} \\[1em] \Rightarrow 0.7002 = \dfrac{AD}{BD} \\[1em] \Rightarrow BD = \dfrac{AD}{0.7002} \\[1em] \Rightarrow BD = \dfrac{20}{0.7002} \\[1em] \Rightarrow BD = 28.563 \text{ m}.$

In ∆ACD,

$\text{tan 42°} = \dfrac{\text{Perpendicular}}{Base} \\[1em] \Rightarrow 0.9004 = \dfrac{CD}{AD} \\[1em] \Rightarrow CD = AD \times 0.9004 \\[1em] \Rightarrow CD = 20 \times 0.9004 \\[1em] \Rightarrow CD = 18.008 \text{ m}.$

From figure,

BC = BD - CD = 28.563 - 18.008 = 10.55 meters.

Hence, BC = 10.55 meters.