CommonWordProblems(4)

　　A population of bacteria grows by 35% every hour. If the population begins with 100 specimens, how many are there after 6 hours?

　　The question, with its growing population of bacteria, makes it quite clear that this is an exponential growth problem. To solve the problem, you just need to plug the appropriate values into the formula for a repeated percent increase. The rate is .035, the original amount is 100, and the time is 6 hours:　　Simple Exponential Decay Problem

　　A fully inflated beach ball loses 6% of its air every day. If the beach ball originally contains 4000 cubic centimeters of air, how many cubic centimeters does it hold after 10 days?

　　Since the beach ball loses air, we know this is an exponential decay problem. The decay rate is .06, the original amount is 4000 cubic centimeters of air, and the time is 10. Plugging the information into the formula:　　More Complicated Exponential Growth Problem

　　A bank offers a 4.7% interest rate on all savings accounts, compounded monthly. If 1000 dollars is initially put into a savings account, how much money will the account hold two years later?

　　This problem is a bit tricky for the simple reason that the interest on the account is compounded monthly. This means that in the 2 years that question refers to, there will be 2　12 = 24 compoundings of interest. The time variable in the equation is affected by these monthly compoundings: it will be 24 instead of 2. Thus, our answer is:　　Here’s another compounding problem:

　　Sam puts 2000 dollars into a savings account that pays 5% interest compounded annually. Chris puts 2500 dollars into a different savings account that pays 4% annually. After 15 years, whose account will have more money in it, if no more money is added or subtracted from the principal?

　　Sam’s account will have $2000　1.0515 ≈ $4157.85 in it after 15 years. Chris’s account will have $2500　1.0415 ≈ $4502.36 in it. So, Chris’s account will still have more money in it after 15 years. Notice, however, that Sam’s account is gaining on Chris’s account.

（责任编辑：申月月）