The original amount of po-239 was 5725 g, How much would be remaining after 4 half lives

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

weathering - colorado river carves land in arizona to form the grand canyon. cracks form in a sandstone wall surrounding a house. a marble statue near the science museum is changing color due to rainfall

erosion - after a rainstorm, soil and rock piees litter the roadways. the sand dunes in the sahara desert are changing shape.

here's your answer.

it is called the mitotic spindle