Lesson 10

Estimation

Estimation

• In real life, an exact answer is not always necessary. Sometimes an estimated answer is all you need.

• Rounding is often a good way to estimate an answer.

Lets take a look

• Last year, Kevin earned $204 per week at his job. This year, he got a raise of $27 per week. What is a good estimate of the percent increase in his weekly pay? – What would you estimate? – Lets find what is close to 204 and 27. – Now we can find the percent increase. – His weekly salary increased by ABOUT 15%.

Next example

• Joanna figures that she saves 27% on groceries when she shops at the Warehouse Store instead of at Pete’s Supermarket. Last Thursday, she spent $83 at Pete’s Supermarket. If she had shopped at the Warehouse Store, approximately how much money would she save?– What common fraction has a value close to 27%?

Well, 27% is close to 25% which is ¼.– Round $83 to the nearest $10. – Now, ¼ of 80 is…. How much would she save?

One more example

• A rectangular plot of land has dimensions of 148 feet by 67 feet. Vince estimated the area of the plot of land by rounding the numbers and multiplying. Area=length x width = 150 x 70= 10,500 square feet. What is the percent error of his estimate?– Percent error? That is where we find the percentage

of the difference between the correct answer and his answer.

– Find the real area. Subtract from actual. Put your answer over the actual area of the plot.

Try some on your own

• On Monday, about 19% of the 495 students enrolled at a middle school went on a field trip. What is the best estimate of the number of student who went?

• Alfredo has a spool with 26.87 centimeters of wire. About how many pieces of wire, if each is 2.85 centimeters, can he cut?

• A rectangle is 8.95 centimeters long and 0.75 centimeters wide. Caryn estimates its area as 9 square centimeters. What is the percent error?

You know the drill

• Amy saves $196 each week and deposits it in the bank.– Estimate the total amount of money Amy will

have saved at the end of 52 weeks. – Find the actual amount Amy will save by the

end of the 52 weeks.– How accurate was your estimate? Make sure

you use percent error in your explanation.