Numbers are one of Indiana’s most important iconographic themes, and the artist explored both their formal and symbolic meanings. In Four Numbers Summing Thirty, as in his 1962 Polygon series, each numeral is placed inside a polygon of the corresponding number of sides (for example the eight is placed inside the octagon). The four numbers in the work are consecutive ones which equal 30, a number that refers to the number of pieces of silver that Judas received for handing over Jesus to the chief priests.  The painting is thus linked to the theme of the crucifixion, which Indiana most famously depicted in his monumental drawing Stavrosis (1958).
 Susan Elizabeth Ryan, Robert Indiana: Figures of Speech (New Haven: Yale University Press, 2000), p. 156.