Answer:
The amount of loan ($20000) after 48 months with 1.5% (0.015) interest rate is calculated by:
A = 20000 x (1 + 0.015/12)^48
= 21235.94$
Monthly payment of Grace is calculated by:
M = A/number of month
= 21235.94/48 = 442.4 = ~444$
Hope this helps!
:)
P.S: I did try the amortization formula, but something wrong with my calculator. I used other formula instead :)