A radioactive isotope has a half-life of 10 days. If today there are 125 g of it left, what was its original weight 40 days earlier 

(a) 600 g (b) 1000 g (c) 1250 g (d) 2000 g 
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D. 2000g 

At the 40th day remaining isotope would be 125g

Since it's half life period is 10 days , on 30th day it would be double the 125g, i.e. 250g.

And on 20th day it would be double the 250g, i.e. 500g. And on 10th day it would be double the 500g i.e.1000g.

And before the radioactive decay the weight of isotope on the first Day will be double the 10th day, i.e. 2000g
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2000g radio activ isotope have 10 day T1/2. So 40 days have a four T1/2 answer is 2000g.

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Half life = 10days 

The formula which is, 1/(2)^n × original weight = Weight left today. 

To obtain the value of

n = divide no of days by half life

ie.,n= 40/10= 4


So, 1/(2)^n × original Weight = 125g

1/(2)^4 × Original weight = 125

Original weight = 16×125 = 2000g

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