According to the graph, how long is the half life of the original substance? *

It is around 2.75 units of time.

Explanation:

Half life defines the time that a quantity of a sustance reduces to it's half.

So if we initially have a mass A of a given substance, after the half life time, we will have a mass of A/2.

In our graph we can see that at time t = 0, we have 100000 atoms, the half life time will be the time at witch we have 100000/2 = 50000 atoms.

You can see that this happens at something between 2.5 and 3 units of time.

If you llok at the time t = 6, we have 30000 atoms, so now we need to see at what time the number of atoms is around 15000 atoms.

this happens at time betwen 8.5 and 9

and 8.5 - 6 = 2.5

9 - 6 = 3

So the half time of the substance is between 2.5 and 3 units of time, we can select the average and say that the half life time is 2.75 units of time