Which number will replace the question mark (?) in the following series?
40, 80, 200, 600, 2100, ?
$$40\times2 = 80$$
$$80\times2.5 = 200$$
$$200\times3 = 600$$
$$600\times3.5 = 2100$$
$$2100\times4 = 8400$$
Hence 8400 will come in place of the question.
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