After the first three days, the sample size will be half of the initial; therefore, 400g/2= 200g. However, it is asking how long will it take to decay to 100g, so we will take 200g/2= 100g, which will take another three days. It will take 2 half-lives, which will encompass 6 days.
9.7 is the sum of both paints used to pain her bedroom walls. If we know that 3.5 of the 9.7 was blue, we can safely assume that the remainder of the paint was white, thus 9.7 - 3.5 = 6.2
