This was hard... I knew the answer is 0.1 (%10,) but I didn't know why I came to that conclusion. It's been too long since I solved a problem like this. I had to research it, and being OCD-driven, I spent too much time trying to make sure my answer was correct.
If we have A & B, as probabilities of two mutually exclusive events. C being the probability that either A or B occurred: C = A + B.
%60 = %50 + X
X = %60 - %50
X = %10
I might be wrong though, it's been long since I used the simple probability equations like this one.