I am trying to solve all "easy" problems within a reasonable timeframe and then move on to "medium" and finally "hard" ones.

While the easy ones don't usually give me trouble I tend to stuck on those which can be normally be solved by using a closed mathematical solution (most of the times are also one-liners). They really break my confidence as they're outliers in term of time taken and overall comfort around the solution.

Any tips on how to recognise one and how to solve them?