26. 16 grams of radioactive material decays into 8 grams in 10 years. How long will it take to decay to 1 gram?
Ans: 70 yrs

• Ans. 40years
  
  This radioactive material has half life after 10 years.
  
  16 grams decays into 8 grams in 10 years.
  Then 8 grams decays into 4 grams in next 10 years.
  Then 4 grams decays into 2 grams in next 10 years.
  Then 2 grams decays into 1 gram in next 10 years.
  Thus 16 grams decays into 1 gram in 40 years.
• 12 years agoHelpfull: Yes(56) No(15)
• it is not mentioned that it will decay in exact half so we cant say 40 years.
  answer will be 70 year.
  
  16 gm reduced to 8gm in 10yrs
  so 8 gm get decayed
  for decaying 1 gm it takes 10/8 yrs
  for decaying 8 gm it takes 10/8*8
  
  remaining 8 gm
  need to decay 7 to get 1 gm remaining
  for decaying 7 it takes
  10/8*8*7
  =10*7
  =70 years
• 9 years agoHelpfull: Yes(5) No(2)
• Ans will be 80 years.
  Radioactive material decays 8 grams in 10 years. Then,
  4 grams in 20 years
  2 grams in 40 years
  1 gram in 80 years.
  
• 12 years agoHelpfull: Yes(4) No(35)
• ans:70
  16gm decays into 8gm in 10y
  so 8gm decays into 4gm=8/2=4*10=40
  4gm decays into 2gm=4/2=2*10=20
  2gm decays into 1gm=2/2=1*10=10
  so 40+20+10=70
  
• 9 years agoHelpfull: Yes(4) No(0)
• 16---------8------>10
  8----------4------>10
  4----------2------->10
  2----------1------->10
  
  so ans is 40yrs
• 10 years agoHelpfull: Yes(3) No(1)
• Ans . 70 yrs
  
  16 gms to 8 gms in 10 yrs
  so.
  1 gms = 10/8 yrs
  4 gms = 4*(10/8) = 5
  2 gms = 2*(10/8) = 5/2
  1 gms = 1*(10/8) = 5/4
  
  on adding
  5+(5/2)+(5/4)
  = (40+20+10)/8
  = 70/8
  
  so it takes 70 yrs to decay 8 grms to q grms
  
• 9 years agoHelpfull: Yes(2) No(0)
• Rate of decay is directly proportioal to mass present at that time.
  dm/dt =-k*m (m= mass)
  by integrating, we get,
  
  ln(mi/mf)=kt+constant mi=initial mass and mf=final mass
  
  at t=0,mi=mf so constant=0
  and t=10years, mf=8 so, k=ln(2)/10
  
  now mf=1
  so, ln(16)=ln(2)*t/10
  hence time=40years !!
  
• 8 years agoHelpfull: Yes(1) No(0)
• 10y 10y 10y 10y
  16---------8 -------------4---------------2---------------1
  total no.of years =40years...
  I think the answer is wrong...
• 9 years agoHelpfull: Yes(0) No(0)
• i think 18-19 yrs
  
• 9 years agoHelpfull: Yes(0) No(4)