Maths GCSE question help

Hi, on my mocks in paper 1 (no calculator) I had a question near the end were it was just a non right angle triangle. with 3 given lengths only . And we were told to find the angle ABC . I knew I had to use the cos rule . I was able to re arrange the equation and got calculated it, but for the last step you have to do the inverse of cos-¹(X) . How am I meant to do inverse non calculator?¿?

Presume its a known value so 1/2 or sqrt(3)/2 or ... ? Could you post the question / your working?

My value was 1/2

For gcse, its one of the angles youre expected to know the cos, sin and tan values for. So
cos(B) = 1/2
B = ....
Its obviously one of 0,30,45,60,90.
Its one way of testing/examining those facts.