The radioactive isotope of lead, Pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. If 1 gram of this isotope is present initially, how long will it take for 90% of the lead to decay? (Round your answer to two decimal places.)

It will take 10.96 hours for 90% of the lead to decay.

Step-by-step explanation:

The radioactive isotope of lead, Pb-209, decays at a rate proportional to the amount present at time t

This means that the amount can be modeled by the following function:

In which A(0) is the initial amount and r is the decay rate.

Has a half-life of 3.3 hours.

This means that . We use this to find r.

So

How long will it take for 90% of the lead to decay?

This is t for which , that is, 100 - 90 = 10% of the initial amount.

It will take 10.96 hours for 90% of the lead to decay.

  or  

step-by-step explanation:

if the four friends split the cost, p, of the pizza evenly, then each friend pays one-fourth of the cost of the pizza.

the expression is   or  

costs the what? what is the cost of the pants?

step-by-step explanation:

if it is $10, then the price is 14.

take 40% of the cost, and then add the cost.

40% of 10 = 4

4+10 = 14.