How many moles of NaBr can be formed from 2 moles of AlBr3?

Amount of element A remaining is 25g

Explanation:

Half-life of an element is the time require for it to reduce to half of its original size.

Given that the half-life of Element A is 10days.

For a 200g ,at the end of first 10days ,it would have reduced to 100g( its half)

At the end of another 10days ,100g would have reduced to 50g(its half)

At the end of another 10days,50g would have reduced to 25g(its half)

Therefore at the end of 30days(10+10+10) the amount of Element A remaining is 25g.