When you are left with 6.25 mg of the radioactive isotope, how many minutes would have passed from time equals zero?

NOTE: Question is incomplete. Problem is solve considering that initial amount of radioactive nuclei is 100 mg and half life is this radioactive nuclei is 100 mins.

Consider that initially 100 mg of radioactive material is present.

Radioactive material obeys 1st order dissociation kinetics.

Now, when the concentration radioactive nuclei is reduce to half of initial amount, it is referred as half life of reaction.

 SO, if initial concentration of reactant is 100 mg, after 1st half life, concentration of radioactive nuclei will be reduced to 50 mg.

After completing 2nd half life, concentration of radioactive nuclei would be depleted to 50/2 = 25 mg.

On pass through next half-life, amount of radioactive nuclei left in system will be 25/2 = 12.5 mg.

Finally, at 4th half life, amount of radioactive element left in system will be 12.5/ 2 = 6.25 mg

Thus, if 6.25 mg compound is left in system, system would have passed 4 half life. 

Now, if half life is radioactive nuclei is 100 mins, Then, it would require 4 X 100 = 400 min for reducing the amount from 100 mg to 6.25 mg