After three half-lives, a sample contains 20 grams of parent isotopes. how many grams of the parent isotope were present to start?

160 g

Explanation:

To calculate we use the formula:

where,

a = amount of reactant left after n-half lives = 20 g

= Initial amount of the reactant = ?

n = number of half lives  = 3

Putting values in above equation, we get:

alfred von schlieffen, german army chief of staff, was given instructions to devise a strategy that would be able to counter a joint attack. in december, 1905, he began circulating what later became known as the schlieffen plan. schlieffen argued that if war took place it was vital that france was speedily defeated.

c is the answer because when something is left after something evaporates that shows the chemical change.