Question

An opaque container is 15 cm deep. Itcontainsonly a single coin. When looking into the container at aviewingangle of 50^o relative to the vertical side ofthecontainer, you see nothing on the bottom. When the containerisfilled with water, you see the coin (from the same viewingangle)on the bottom of, and just beyond, the side of the container.Howfar is the coin from the side of the container?

I see the formula and understand but I do not get why wearecalculating from a 90 degree angle and why we suddenly switchedtotangent in calculating the answer...