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(c) 20 m

Let the height of the triangle be $x \mathrm{~m}$ and its base be $4 x \mathrm{~m}$ respectively.

Then, area of the triangle $=\left(\frac{1}{2} \times 4 x \times x\right) m^{2}$

$=2 x^{2} \mathrm{~m}^{2}$

But, the area of the triangle $i s 50 \mathrm{~m}^{2}$.

$\therefore 2 x^{2}=50$

$\Rightarrow x^{2}=\frac{50}{2}$

$\Rightarrow x^{2}=25$

$\Rightarrow x=\sqrt{25}$

$\Rightarrow x=5$

$\therefore L$ ength of its base $=(4 \times 5) \mathrm{m}=20 \mathrm{~m}$