Aradioactive isotope has a half-life of ten days. how much of a 100 gram sample will be left after 20 days?

It's always nice to make a table and do things visually for these kinds of problems. It causes for less mistakes and more accuracy. I also find making a table is easier to understand, than doing it out mentally.

Now remember when we start our percentage always starts at 100 and amount always starts at what is specified. In this case, we have an 100 gram sample.

Our half life and time is always ZERO at first. This is important to remember. We start at zero, because we are starting from when the sample first starts to decay.

A half-life is the time it takes for half of the sample to decay. So we simply divide the amount of the sample in half, then calculate the percent for it.

In the image I provided of the table you'll see the amount drop from 100 to 50 to 25.
For every half-life it divides in half. If you notice, that percent does the same.

So how do I figure out what unit to use for time?

I'll show yah! It's wicked simple! The problem tells you!
It says the sample has a half-life of 10 days. So for every half-life 10 days go by.

Our answer to the question if we look at the table is 25 grams. There will be 25 grams left after 20 days!

I hope this helps! If you have any questions let me know!

The answer is option a.