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 .

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1) when you rub your hands together quickly, the kinetic energy is converted to mechanical energy.2) thermal energy from the sun travels to earth as infrared waves.3) energy is transferred through the matter of space by electromagnetic waves. 4) when a match is burned, chemical potential energy is converted to thermal and radiant energy. 5) seismicwaves are caused by back and forth movement , or vibration, of an object. 6) mechanicalenergy in an engine changes thermal energy into kinetic energy to make a car move.