[piqwhqfy] 102 and 221 for mental arithmetic

102 = 17*6 and 221 = 13*17 have low digit weight, so it is easy to add or subtract those multiples of 17 and 13 without many carries.

17*17 = (4*17) + (13*17) = 68 + 221 = 289, if you already knew 4*17 = 68.

possibly useful for memorizing composites.

can the technique be generalized to decrease combinational depth of ripple carry adders?