Question 1077463
This is ordinary annuity problem.
We have 
FV = PMT[(1+i)^n-1]/i
Here PMT = 800, i = 4.2%/12 = 4.2/1200, n = 25*12 = 300
Hence, FV = 800[(1+4.2/1200)^300-1]/(4.2/1200)
= 423409.65