[fullwit]

Presumably, there had been discussion before about Halfwit getting to Oxford despite his disorder and Sree was using it as a backhanded compliment. "What's 15 x 15" was just Sree being childish about his own academic ability. Dyslexia has nothing to do with numbers. When Davina asked Sree about the incident he did seem genuinely confused as to why she was asking him, rather than playing the innocent.