the half-life of an element x is is 5 days, if we have 5g of x initially, what is the mass x after 20 days

the half-life of an element x is is 5 days, if we have 5g of x initially, what is the mass x after 20 days

After 5 days there is 5/2 = 2.5 g left.

Another 5 days there is 2.5/2 = 1.25 g left.
Another 5 days there is 1.25/2 = 0.625 g lef.t
Another 5 days there is 0.625/2 = 0.3125 g left.
After 20 days there is 0.3125 g left.