The d20 roleplaying system allows you to “take 10” on a skill roll for which you would normally roll a d20. This means you can settle for an “average” result of your skill check, without bothering to roll to see if you do any better. Taking 10 also means there is no risk of performing badly at the skill, which may carry a penalty if, for example you roll the die and roll a 1.
Another option is to “take 20”, which means you spend a long time trying the skill repeatedly, until you eventually achieve a result equivalent to rolling a 20. This can be used, for example, to pick locks, where repeated failures carry no penalty apart from the time taken to try again. Eventually you’ll get it right and, if the lock is at all within your capability of opening, you open it. If you take 20 on a task for which there are penalties for failure, you automatically trigger those penalties, since it assumed that you actually fail your skill check several times before succeeding. So you can’t take 20 to sneak past a sleeping dragon, for example, unless you don’t mind waking the dragon.
For better or worse, there is no mechanic for “taking 51” on a d20 skill roll.