Why do solutions to problems feel simple (or obvious) once we are given them?
The closest one that comes to mind is called the Hindsight Bias. Basically this means that things seem to be "more obvious" than it actually is--usually after the event had just happened. 

Definition:

Hindsight bias is a term used in psychology to explain the tendency of people to overestimate their ability to have predicted an outcome that could not possibly have been predicted. In essence, the hindsight bias is sort of like saying "I knew it!" when an outcome (either expected or unexpected) occurs - and the belief that one actually predicted it correctly.

Usually coming up with the idea is difficult, for instance solving a math problem. The challenge is finding the right mental model to approach the problem, but once the solution or approach is presented to you, it may be easy to accomplish and you'd feel it was obvious.

Or it could be really just that predictable or that you're natural/good at the subject (e.g. Math) to begin with and it is easy for you.