The following expression represents the concentration, in microbubbles per milliliter, of the microbubble suspension remaining in a client t seconds after the technician has begun the ultrasound process.

100,000,000 ⋅ 0.5^(t/120)

Which expression shows the multiple of the concentration remaining each minute as a constant or coefficient?

You didn’t give me answer choices, but it seems like the key here is converting seconds to minutes. If you have seconds and you want minutes, you divide by 60, so you’re looking for something that has \dfrac{t}{60} in it.

Off the top of my head, maybe this?

100,000,000 \times 0.5^{\left(\frac{t}{60}\times\frac{1}{2}\right)}