At level 20 in LOTRO, you should work on Dark blue, white and yellow quest only (exceptions is when deed farming).
1. LOTRO gold in quest that are dark blue first then the whites. DO NOT cash in yellow are orange quests.
2. Wait for your character to get to the LOTRO gold. A yellow level 15 quest doesn't give you the same bang for your buck at level 13.
3. You get roughly the exact same xp but 800xp at level 15 is better than 800 xp a level 13. The reason is that there are always more quest at the lower levels than at the higher levels so if you cash in all your quest when they are yellow you will run out of quests when you get to the high teens.
4. Do quests that are dark blue, white and yellow. Cash in the blue and whites and wait for the yellow quest to turn white. (the exception to the yellow or higher quests is when you are level 19 cash in all quest at level 19 your goal is level 20 so waiting until level 20 isn't going to help you anymore for the title)
5. When you do dark blue, white and yellow quests you are dealing with mobs that you can handle and kill solo. But be aware of class limitations a champion can handle multiple mobs better than a hunter. learn your classes limitations.
6. A good rule of thumb is if you fight a battle and your health is down to 50% then that is the max number of mobs you can handle safely and be assured you can win almost always. It also means you can likely handle 1 more mob but the battle will be close and if you make a mistake or are unlucky you could die, So be aware. For example you hunter fights 2 mobs and is reduced to 50% health this means you are likely able to handle these types of mobs in groups of 2 or less. It also means that you should be able to take 3 but there is a chance things could go wrong and you could die. So in battle with 3 or more you have to be sure that you winning and don't need to run. Obviously each class is different and where a champion might be able to take on 3 or 4 no problem and loremaster will not be.
7.Use the rule of 50% to judge how many mobs you can handle.