# Calculating Half-Life: 0.48g to 0.003g

• Zoheb Imran

## Homework Statement

The Half-life of a radioactive substance is 5 hours. A sample is tested and found to contain 0.48g of the substance.
How much of the substance was present in the sample 20 hours before the sample was tested?

## The Attempt at a Solution

i did like this...

20/5 = 4 half life

0.48---->0.24--->0.12--->0.06--->0.003g amount of substance was present.

am i right?

How much of the substance was present in the sample 20 hours before the sample was tested?

Right idea, but you want the other direction in time...

why the other direction? can you please explain? that's where i am having problem...in calculating...

why the other direction? can you please explain? that's where i am having problem...in calculating...

You are calculating forward in time when you used the half-life. So you are figuring out how much of the radioactive material is left after it was sampled. The question asks you to figure out how much of the material was present before the sample was made...

thanks!