How to calculate opposite direction angle

I searched but couldn't find on Google:

My question is, how do I find the opposite direction of an angle, for example 170 degree, how do I calculate the opposite direction in degrees?

Thanks in advance.

Solution 1:

If you are given an angle $\alpha$, the oppossite angle would be $\alpha+180$. If you need to remain in $[0,360]$ then, you should take $(\alpha+180)\ mod\ 360$ (what in this case it is simply taking $\alpha-180$ if $\alpha+180\ge360$)

Solution 2:

In aviation the 200/20 rule can be used. If your current heading in degree is below 180° e.g. 80° then you add 200 and subtract 20. 80+200=280. 280-20=260. If your current heading in degrees is above 180° then subtract 200 and add 20. E.g. 240-200=40. 40+20=60