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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

= 1x1/8

= 1/8
1/8  fraction of the initial quantity of the isotope will be left behind.