Just for completeness (and correcting minor miscalculations of previous poster), here is a table with probabilities:

Code
target  normal  adv.    abs.diff rel.diff
1	100.00%	100.00%	0.00%	0.00%
2	95.00%	99.75%	4.75%	5.00%
3	90.00%	99.00%	9.00%	10.00%
4	85.00%	97.75%	12.75%	15.00%
5	80.00%	96.00%	16.00%	20.00%
6	75.00%	93.75%	18.75%	25.00%
7	70.00%	91.00%	21.00%	30.00%
8	65.00%	87.75%	22.75%	35.00%
9	60.00%	84.00%	24.00%	40.00%
10	55.00%	79.75%	24.75%	45.00%
11	50.00%	75.00%	25.00%	50.00%
12	45.00%	69.75%	24.75%	55.00%
13	40.00%	64.00%	24.00%	60.00%
14	35.00%	57.75%	22.75%	65.00%
15	30.00%	51.00%	21.00%	70.00%
16	25.00%	43.75%	18.75%	75.00%
17	20.00%	36.00%	16.00%	80.00%
18	15.00%	27.75%	12.75%	85.00%
19	10.00%	19.00%	9.00%	90.00%
20	5.00%	9.75%	4.75%	95.00%

target - min number you have to roll to pass
normal - probability of passing without advantage
adv. - probability of passing with advantage
abs.diff - difference between probabilities of passing with and without advantage
rel.diff - relative improvement in chance to pass ( = abs.diff / normal )