What is the next serie?
1,3,6,10,15,______

Answer: C. 21

The difference between the two consecutive term follows the pattern 2,3,4,5 and so on,The difference between 10 and 15 is 5 .So the next term after 15 should be 6 more than 15 ,which is 15+6=21.Hence the correct answer is option c.

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