$100?
100+80=180
180-23=157
157-90=67
Hope this helps !
Answer:

Where
represent the half life and the intial amount would be 
And we want to find the time in order to have a 95% of decay so we can set up the following equation:

If we apply natural log on both sides we got:

And solving for t we got:

So then would takes about 14.26 hours in order to have 95% of the lead to decay
Step-by-step explanation:
For this case we can define the variable of interest amount of Pb209 and for the half life would be given:

Where
represent the half life and the intial amount would be 
And we want to find the time in order to have a 95% of decay so we can set up the following equation:

If we apply natural log on both sides we got:

And solving for t we got:

So then would takes about 14.26 hours in order to have 95% of the lead to decay
C-4 miles because he’s running from the bike
Undo the subtraction of 4 by adding 4 to both sides




This means v is less than or equal to 24.
In other words, v can be equal to 24 or it can be smaller than 24