Sea
X : La longitud del puente.
Resolviendo
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B1%7D%7B5%7DX%2B%20%5Cfrac%7B1%7D%7B6%7DX%2B760%3DX" />
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B6X%2B5X%7D%7B30%7D%2B760%3DX" />
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B11%7D%7B30%7DX%2B760%3DX" />
<img src="https://tex.z-dn.net/?f=11X%2B22800%3D30X" />
<img src="https://tex.z-dn.net/?f=22800%3D19X" />
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B22800%7D%7B19%7D%3DX" />
1200 = X = = = > La longitud del puente.
Comprobación :
<img src="https://tex.z-dn.net/?f=%20%5Cfrac%7B1%7D%7B5%7D%281200%29%2B%20%5Cfrac%7B1%7D%7B6%7D%281200%29%2B760%3D1200" />
240 + 200 + 760 = 1200
1200 = 1200
MUCHA SUERTE!