How many half-lives would have to pass before the radioactivity of an isotope falls to less than 1% of its original value?

In what direction does the medium move relative to the direction of the wave? explain.

Describe uniform velocity and variable velocity in short

Abucket full of water weights 4 kg and water well is 10m deep. a girl draws water from the well. it takes the girl 3 minutes to draw a bucket full of water from the well. what is the power of the girl?