Find the one which does not belong to that group ?
Answer: C
20 = 4⊃2; + 4, 42 = 6⊃2; + 6, 58 = 7⊃2; + 9, 72 = 8⊃2; + 8 and 90 = 9⊃2; + 9.
20, 42, 72 and 90 can be expressed in n2 + n form but not 58.