The half-life for such a substance is the time 
required to have the amount (in grams 
) at time (in seconds 
), 
, decay to half the starting amount 
. In other words, for krypton-91, after 10 seconds we would have 
. We can use this information to solve for 
, then plug in various values of to find the amount of krypton-91 that's left...
But we don't need to! We already know it takes 10 seconds for half of the substance to decay. So if we start with 16 grams, then after 10 seconds, 8 grams will remain.
10s later (after a total of 20 seconds), 4g will remain.
10s later (30s), 2g will remain.
10s later (40s), 1g will remain.
10s later (50s), 1/2 g will remain.
The Number of text messages sent in a month is 1.68 × 
Step-by-step explanation:
Step 1 :
Total number of teenagers in United States = 21 million
Average text messages sent per month = 800
We need to find the number of text messages sent each month
Step 2 :
The number of text messages in a month can be computed by taking the product of the total number of teenagers and average messages sent per month
Therefore
Number of text messages that is sent in a month = 21 million × 800
= 21 × 
× 800
= 168 × = 1.68 ×
Step 3 :
Answer :
The Number of text messages that is sent in a month is 1.68 ×
Answer:
Total cost = f + ad
Step-by-step explanation:
Given:
Number of days = d
Fixed charges = f
Cost per day = a
Find:
Equation
Computation:
Total cost = fixed cost + (Number of days)(Cost per day)
Total cost = f + ad
