The half-life of a radioactive kind of ruthenium is 40 days. If you start with 240 grams of it,

how much will be left after 120 days?

Answer: 30g

Step-by-step explanation:

In this question (t½) of isotope is 40 days, which means that after 40 days half of the sample would have decayed and half would be left as it is.

After 40 days (first half life) 240 / 2 = 120 g decays and 120 g remains left.

After another 40 days (two half lives or 80 days) 120 / 2 = 60 g decays and 60 g remains left.

after three half lives or 120 days, 60/2 = 30g decay and 30 g of the isotope will be left.