to skip or not to skip probability & combinations

Brent, would skipping probability and combinations, and perhaps, coming back to them later in my studies be much of a big deal? In other words are they seldom tested? I haven't looked at them using the study guide because i'm taking a some more time solving upper level word problem's, and I think that learning probabilities and combinations well would be an inefficient use of my time. I'm worried though that I might be to lax with that mindset. Perhaps you could offer some advice because I'm not so sure I'm giving myself the right instruction... Thank you.