Now the half life of the isotope is given 2 hours. After 6 hours the no of half lives will be 3.
So the The amount of radioactive isotope remaining can be calculated as:
Nt = No x 1/2^n where, Nt is the amount of radio isotope remaining. No is the original amount of radio isotope.
N is the no of half lives.
Now let the original amount of isotope is one, then by using the formula we can calculate the remaining of isotope as:
Nt = 1x1/2^3