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?

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.