The [half-life](https://en.wikipedia.org/wiki/Half-life) of a radioactive substance is the time it takes (on average) for one-half of its atoms to undergo radioactive decay.
# Task Overview
Given the initial quantity of a radioactive substance, the quantity remaining after a given period of time, and the period of time, return the half life of that substance. 
# Usage Examples
```if:csharp
Documentation:
Kata.HalfLife Method (Double, Double, Int32)
Returns the half-life for a substance given the initial quantity, the remaining quantity, and the elasped time.
Syntax
public
static
double HalfLife(
double quantityInitial,
    double quantityRemaining,
int time
    )
Parameters
quantityInitial
Type: System.Double
The initial amount of the substance.
quantityRemaining
Type: System.Double
The current amount of the substance.
time
Type: System.Int32
The amount of time elapsed.
Return Value
Type: System.Double
  A floating-point number representing the half-life of the substance.
```
import math
def half_life(initial, remaining, time):
    return time / math.log2( initial / remaining )
import math
def half_life(initial, remaining, time):
    return time / math.log2( initial / remaining )