Bangladesh was dismissed for 70 in 15.4 overs after fast bowler Mustafizur Rahman had grabbed 5-22 but still couldn't stop New Zealand from scoring a competitive 145-8.
New Zealand's fourth successive victory was halted for 11 minutes when one of the light towers at Eden Gardens went off with Bangladesh at 45-6 in 11 overs before Grant Elliott (3-12) and Ish Sodhi (3-21) finished off the match.
New Zealand topped its group with eight points after earlier beating hosts India, Australia and Pakistan. Bangladesh lost all its group matches that included a last-ball, 1-run loss against India.
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