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