tan(300/x) = 15.9*

If I had to do it by hand and make a guess, I would start by estimating the 15.9 in terms of pi.

There are only a few values of tangent that people can be expected to remember... you would need a Taylor Series to solve tangent by hand. I don't think any employer is that cruel.

tan=sin/cos and there are a few values of sin and cos that you should kinda know also.