By Suman Patra

**Answer:**

The half-life of a radioisotope is the time required for half the atoms in a given sample to undergo radioactive, or nuclear, decay.

Initially, at time t = 0, the sample is 100%.

After 50 days, only half the original amount of element will remain:

½ x 100% = 50%

After another 50 days, only half of this amount of will remain:

½ x 50% = 25%

After another 50 days, only half of this amount of strontium will remain:

½ x 25% = 12.5%

This can be represented in the form of a table:

Number of Half-lives |
Time (Days) |
% of Radioactive element remaining |
% of Radioactive element decayed |

0 | 0 | 100 | 0 |

1 | 50 | 50 | 50 |

2 | 100 | 25 | 75 |

3 | 150 | 12.5 | 87.5 |

Hence, we can see that there are 3 half lives required for the element to become 12.5% of the original amount.

In case you need to calculate the percentage remaining after some other number of days or years or hours, you can use the formula;

N_{t} = N_{o} X (0.5)^{Number of half-lives}

Where:

N_{t} = Amount of radioisotope remaining

N_{o} = Original amount of radioisotope

Number of half-lives = Time ÷ Half-life

N_{o} = 100%

(Time is the number of days or years or hours given).