Here we are finding x, given the angle and adjacent side. To find x we will use the function cos as cos = a/h.
So let's do cos(35°) = 15cm / x
cos(35°) = 0.8 (1 dp)
x = 15 cm / cos(35°)
x = 18.3 cm (1 dp)
Answer: Choice A)

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Work Shown:

This is not the only way to solve for H.
6 foot makes 4.5 foot shadow
1 foot makes=4.5/6
so 50 foot =4.5/6*50 = tree makes 37.5 foot shadow
It would be B. because the rise over run is 31.25 over 5 which is divided to 6.25 which is rounded to 6.3