I dont understand how the solution is obtained..I try to use present value of an annuity immediate but I dont see how to get the answer
The answer is,
Can we just use the intrest given? since the payment period and intrest conversion align? What does it mean by a nominal annual rate compounded every half month, isnt that ussually given with a superscript?
There are $24$ half-months in a year, so the interest every half-month is $\frac 6{24}\%=\frac 14\%$. You can use your usual formula with that or make a spreadsheet showing the interest, payment, and balance every half-month. Interest is often quoted as a nominal annual amount which must be scaled to the payment period.
Alternately, nothing in the problem statement prohibits saying $n=1$ and the only payment is $(1+0.0025) \cdot 2,000,000=2,005,000$, which is duly no less than $20,000$