Which number completes the series: 1, 3, 6, 10, 15, ???

21

Step-by-step explanation:

1 + 2 = 3

3 + 3 = 6

6 + 4 = 10

10 + 5 = 15

15 + 6 = 21

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1)is not similar since corresponding sides are not in the same ratio

2)are similar since corresponding sides are in the same ratio and corresponding angles are equal

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24

step-by-step explanation: