Compounded quarterly means every 3 months;

so there are 18 / 3 = 6 compounding periods: understand?

During 1 year there are 4 quarters; so rate evety quarter = 5 / 4 = 1.25%.

So the $50000 will accumulate to : 50000*(1 + .0125)^6

Fyi, standard formula is : A(1 + i)^n ; got that?