Question

if ranga wanted to marry a matured girl, then why did he marry a 11 year old girl???

Answer 1

yes...its true that ranga wanted to marry a matured girl...but he married an 11 year old girl because though ratna was a teen, she was quite mature, she could cook, she could sing and she could make a perfect housewife.
   moreover, ranga developed a liking for her at first sight....so he married her.

Answer 2

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The narrator had made up his mind that he would get Ranga married early. First he brought Ranga and Ratna face to face to arouse his interest in her. In order to test the strength of his emotions, he told Ranga that she was married a year ago. The shrivelled face of the young man betrayed his feelings