Sentence:  These cells were thawed at 37 ` C for 30 min and were deglycerolized by alternately centrifuging and mixing with descending concentrations of glycerol solutions ( <WSD>20</WSD> , 18 , 10 , 8 , 4 and 2 % ) . 
Meaning of the word marked with <WSD> tags:  denoting a quantity consisting of 20 items or units