Based on the wording of the question it seems to be a percent difference problem. 50% first day to 60% the second day. We're not calculating probability that the picture will actually change.
Increase = New Number - Original Number.
0.1 = 0.6 - 0.5
0.1 / 0.5 = 0.2 x 100 = 20%
20% chance on second day
It's past 5 a.m. here and I used up most of my math juice working on a fan theory about Barney Miller. Yes, I am also still working on part 2 of that theoretical d.buzz post. I can do both.
I blame the Ziggy handlink I have to keep rattling and banging if this probability isn't correct. I bought it from some government auction. Cheap piece of crap.