There is a 30g of be-11 it has a half-life of about 14 seconds how much will be left in 28 seconds

There will be 7.5 g of Be-11 remaining after 28 s.

If 14 s = 1 half-life, 28 s = 2 half-lives.

After the first half-life, ½ of the Be-11 (15 g) will disappear, and 15 g will remain.

After the second half-life, ½ of the 15 g (7.5 g) will disappear, and 7.5 g will remain.

In symbols,

N = N₀(½)^n

where

n = the number of half-lives

N₀ = the original amount

N = the amount remaining after n half-lives