Approximately how many half-lives must pass for the amount of radioactivity in a substance to decrease to below 1% of its initial level?

7.

Explanation:

The half-life is the time needed to a radioactive element to decay to its half initial concentration.

The first half-life: decay from 100 % → 50 %.

The second half-life: decay from 50 % → 25 %.

The third half-life: decay from 25 % → 12.5 %.

The fourth half-life: decay from 12.5 % → 6.25 %.

The fifth half-life: decay from 6.25 % → 3.125 %.

The sixth half-life: decay from 3.125 % → 1.5625 %.

The seventh half-life: decay from 1.5625 % → 0.78125 %.

So, the substance to decrease to below 1% of its initial level takes 7 half-lives

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.