Question

21. Figure 14-1 shows the decay of a sample of neptunium-238 over a period of 10 days. What is the half-life of neptunium-238?

Answer 1

The half life is the time at which half the sample has decayed and half of it is still around. So you need to look for the 1/2 on the Y axis and see what value of time it corresponds to on the independent axis.

That said, this is not a biology question. It (and those like it) should probably be posted in the physics or math group instead. OK?

Answer 2

^Nice answer. Here's a visual if it helps...your figure probably looks something..vaguely...like this? Exponential decay? You simply find the point where amount A is half...and look at the corresponding time t.