what is the gematriya? what sort of mistake does reuven catch reb saunders in?

The gematriya is a system of assigning numerical value to a word or phrase, and Danny has answered a question that his father asked incorrectly. Danny's father asks to to answer the same question, and Reuven, terrified and astonished that he is being asked to correct a great tzaddik, tentatively points out a mistake in Reb Saunders’s gematriya. Reb Saunders and Danny, along with the entire crowd, are delighted at Reuven’s intelligence.