Sally has invested a constant amount at the end of every six months for the last $40$ years. She has made wise investment decisions and has earned a return of $16\mathrm{.}4\%$ compounded semi$-$annually. Today the value of her savings has reached $$2,500,000.$ What amount has she been investing every half$-$year?

a

$$5,834\mathrm{.}15$

b

$$61,983\mathrm{.}84$

c

$$375\mathrm{.}29$

d

$$91\mathrm{.}66$