A sample of a radioactive isotope had an initial mass of 480 mg in the year 2009 and decays exponentially over time. A measurement in the year 2011 found that the
sample's mass had decayed to 390 mg. What would be the expected mass of the
sample in the year 2016, to the nearest whole number

let's first notice, the center of the hyperbola is at the origin, since the focus points are equidistantly 3 units away from the origin, that also means that the distance "c" from the center to either focus is 3.

it should also be notable that, the traverse axis for this hyperbola is vertical, or namely over the y-axis and therefore, the positive fraction will be the one with y-variable.

the traverse "a" component or distance from the center to either vertex is not 2, is more than 2, so we can safely put it at √5 ≈ 2.24.

the answer is  

d.

21

i did this one. so you should get it right.