5.
a 120.0 g sample of thallium-208 decays. how much of the sample will remain after 3 half lifest

15.0 g

Explanation:

We are given;

Original sample of Thallium-208 as 120.0 g The number of half lives as 3

We are required to calculate the remaining mass after three half lives

We are going to use the formula;

N = N₀ × 0.5^n

where, N is the remaining mass, N₀ is the original mass and n is the number of half lives.

Therefore;

N = 120.0 g × 0.5^3

  = 15.0 g

Thus, the remaining mass of Thallium-208 is 15.0 g

Alternatively;

Since after every half life the mass reduces by half of the original then we can half a chain showing the reduction after three half lives;

120.0 g → 60.0 g (120 × 0.5) → 30.0 g (60 × 0.5) → 15.0 g (30 × 0.5)

Therefore, the remaining mass after three half lives is 15.0 g

Forgot the question hahaha

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.