PV

You do not make it clear whether it is an annuity of 2,500,000 a year for 20 years, or just one flow of 2,500,000 in 20 years time.

Whichever it is, the formula is given at the top of each of the present value tables and so you can calculate it with n = 20 and r = 0.20

(Alternatively, if it is just one flow of 2,500,000 in 20 years time, then you can simply discount for 15 years at 20% and then for another 5 years at 20%. The answer may be slightly different because of rounding, but that does not lose marks in the exam).

It would be very unusual for the examiner to expect you to discount for more than 15 years. At Paper F9 the discounting is generally trivial and is using the tables, because students have already been tested on things like your question, at Paper F2.

At Paper F9 the problem is sorting out the cash flows, not messing around with the discounting which carries very few marks.