Whether a 20degree bag will keep you warm depends on many things:
How accurate is the rating (look for the new European ratings)
What other clothing are you wearing
What do you have underneath you for insulation
How tired, dehydrated, hungry you are
Whether you sleep warm of cold
How humid it is
How windy it is.

My go to bag for three season high mountain is a marmot 15 degree bag because I sleep cold. YMMV
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If I wouldn't eat it at home, why would I want to eat it on the trail?