1. Given that a freshly prepared radioactive isotope has a halflife of 10 days, the percentage of it remaining after 30 days is
A 
30.0 %. 

B 
10.0 %. 

C 
12.5 %. 

D 
72.5 %. 
2. During a second halflife, the original material has decayed
A 
25%. 
B 
50%. 
C 
75%. 
D 
100%. 
1. Answer is C = 12.5%
Since half life period of the isotope is 10 days.
Then After first 10 days, 50% will be decay. then reamining amount is 50% of initial amount.
Then after another 10 days, 50% of remaining isotpe will decay. hence net 25% of initial amount will present.
Then After another 10 days,50% of remaining isotpe will decay. hence net 12.5% of initial amount will present .
Hence 12.5% of initial amount will reamin after 30 days.
2. Answer is C = 75%.
After first half life, 50 % will decay.
After second halflife 50% of remaing isotpe will decay. Hence 25% of initial amount will present. hence 75% of initial amount will decay.
Amount of isotopes present after n consucetive halflife is given by the equation
Where N = amount of isotpes present
N_{0} = Initial amount of isotpe
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