1. How many half-lives must elapse until 86.0% of a radioactive sample of atoms has decayed? (Note that there is no unit for this answer.)

2. A radioactive sample at a lab contains a mixture of ²²³Ra and ⁴⁷Ca. There are initially twice as many calcium atoms as there are radium atoms. What is the ratio of calcium atoms to radium atoms 4.00 weeks later? The half life of ²²³Ra is 11.43 days and that of ⁴⁷Ca is 4.536 days. (Note that there are no units in this answer.)

3. A sample of ¹⁴C has an activity of 1.10 Ci. What is the mass of the sample? The half life is 181000000000 s, and the atomic mass is 14.0032419884 u

4. Nuclear medicine often uses radioactive samples as a tracer to monitor processes inside the body. Suppose a nuclear reactor makes a certain isotope sample with an initial activity of 155 mCi, and the sample gets delivered to a hospital 11.0 hours later, when its activity has become 56.8 mCi. Luckily, the sample remains usable until its activity drops below 10.8 mCi. How long after the delivery is this sample usable?

please I ned help with these questions, with explanation too please.