I’ve often seen ages listed in fractions of a year 1/12, 2/12, etc. but it is not often that I see an age listed as a fraction of the number of days in the month. Maggie Johnson is listed as 20/30 for her age in this 1880 census enumeration from Prairie Township, Hancock County, Illinois. The census states that she was born in May. The date this census was taken was 12 June 1880 and the actual date of the census was 1 June 1880. My guess is that this means she was 20 days old–but May does have 31 days, not 30.
1880 U S Census, Prairie Township, Hancock County, Illinois , page 20D, line 40.
Next step is to see if there is a birth certificate for her. That search should start in the records of Hancock County and expand from there if needed.