6.900x10^10 ÷ 4.000x10^8

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

answer:

technetium is the most reactive. this is followed by silver. the least reactive out of these three elements is strontium.

50.23 i believe is correct but don't quote me on that.