Hi everyone, I am stuck with this question on annuity, which requires some properties I am not sure of.

The question assumes a constant interest rate.

Given that:

1) the PV of an annuity due to in N periods is 12.

2) the PV an annuity due in 2N periods is 21

Compute the present value of an annuity IMMEDIATE in 4N periods.

I am compeltely confused, and do not know where to start, except that (1)*(1+i) = (2)...