Fill in the missing number in the series: 2, 3, 6, 15, __, 157.5, 630

30
37.5
45
52.5

Correct answer: 52.5

Solution

The pattern is multiplication by successive half-integers: 2 d7 1.5 = 3, 3 d7 2 = 6, 6 d7 2.5 = 15. Continuing gives 15 d7 3.5 = 52.5, then 52.5 d7 3 = 157.5 and 157.5 d7 4 = 630. So the missing number is 52.5.

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