Starting with 145 grams of a radioactive isotope how much will be left after 3 half-lives. Calculating Half-Life from a Graph 1. A 800 05 5.
So were starting off with well were starting off with N sub 0 times e to the minus-- wherever you see the t you put the minus 5730-- so minus k times 5730.
Go down half the original count rate and mark it on the graph. So this tells us that after one half-life-- so t is equal to 5730. Tfrac 1 2 t21. This is one half-life has gone by two half-lifes we have an exponent of two three half-lifes we multiply by three.