Verbal Reasoning - Mental Ability Questions

In the following question, select the missing number from the given series.
1, 3, 6, 10, 15, ?

Choose the related word/letters/number from the given alternatives. 19 : 6859 : : 21 : ?

Find the next in this series:
YC3, XF6, WI9, ........

If "α" denotes "subtracted from", "β" denotes "multiplied by", "θ" denotes "added to" and "δ" denotes "divided by", then 10 θ 8 β 4 δ 8 α 9 = ?

If 8α9 = -72, -9α3 = 27 and -6α1 = 6, then find the value of -3α6 = ?

A series is given with one term missing. Select the correct alternative from the given ones that will complete the series.

DE, HI, LM,?

Which number will complete the series?
1,8,27,64,125,216,________

In the following question, select the related number from the given alternatives.
4 : 17 : : 7 : ?