**Answer:**

**The amount of Polonium-210 left in his body after 72 days is 6.937 μg.**

**Step-by-step explanation:**

The decay rate of Polonium-210 is the following:

(1)

Where:

N(t) is the quantity of Po-210 at time t =?

N₀ is the initial quantity of Po-210 = 10 μg

λ is the decay constant

t is the time = 72 d

The decay rate is 0.502%, hence the quantity that still remains in Alexander is 99.498%.

First, we need to find the decay constant:

(2)

Where t(1/2) is the half-life of Po-210 = 138.376 days

By entering equation (2) into (1) we have:

**Therefore, the amount of Polonium-210 left in his body after 72 days is 6.937 μg. **

I hope it helps you!