How do you find the half-life using a graph?

Read off the time it takes for the activity to halve. e.g. decay from 80 to 40. This is the half-life. Then if you can do a repeat read off e.g the time to decay from 60 to 30, and find the average of the half-lives.

On your y axis find look at your highest point, then find half of this. Got to this point on the y-axis and using a ruler draw a line straight across until it hits your graph line (this should be half of the original amount of radioactive isotopes left or half of number of decays). Now draw another line from this point on your graph down to the x-axis (use your ruler again!) - read off the number and this will give you the half life of your substance.