Answer:
A constant with hidden variable being two times on first term increased by three times.
or,
A hidden age of adult twice adding with variable increasing with three times.
So what you asked this is what I got
x(y) = 4.5
If x= 0.5 then
0.5(y) = 4.5
y=9
x(y) = 4.5
10(y) = 4.5
y= 0.45
hope this helps :)
You're starting at 400. Each time you're cutting in half.
So the common ratio is 1/2 which means the equation is

which is in the geometric sequence format. So the answer is choice D