After 6 hours the amount of caffeine 40 mg. After another 6 hours the amount of caffeine is 20 mg. It takes 12 hours for the total amount of caffeine to drop down to 20 mg. the answer is 11 am -B. Hope this helps!
Answer: 
Step-by-step explanation:
Given the following expression:

You need to substitute the given values of "a" and "b" into the expression. Notice that these values are:

Then;

Now you must solve the multiplications:

The final step is to add the numbers. Therefore, you get the following answer:

1/2 because 5/9 is equivalent to 10/18. Half of 18 is 9 and 10 is close to 9 so the nearest benchmark fraction you should round to is 1/2. Hope this helps you!