What is the value of 1st quartile in the data set below? scores on a math quiz: 65, 90, 100, 72, 88, 55, 73

First quartile would 65 because
well first order the numbers around : 55, 65,72,73,88,90,100

the median is 73, the first quartile in the medians of the numbers lower than the whole sets. That means  in the subset of the numbers the median would be: 55, 65, 72. 
crossing out 55 and 72 you have 65.