You are given:

Effective date of a decreasing annuity: $1/1/12.$

Date of the first payment: $\frac{7}{1}\frac{?}{12}$

Frequency of payments: Semiannually

Number of payments: $20$

Semiannually decrease in payment: $$100$

Interest rate: $5\%$ per year, compounded semiannually

Present value $($at $\frac{1}{1}\frac{?}{18})$ of remaining payments after $1/1/18$: $$30000$

Calculate the first semiannual payment $x.$