With a and b? i don't understand how to do it

A).  If the half-life is 40 days, then 5 half-lives = (5 x 40) days.

b).  If 5 half-lives have passed, then the radioactive portion of
the sample has been cut in half five times.

The amount that remains is  (1/2)⁵ of the original amount.

That's (the original amount) x (1/2 · 1/2 · 1/2 · 1/2 · 1/2)
or
           (the original amount) / 2⁵
or
           (the original amount) / 32 .

Uncle grandma