3rd Grade - Estimate To See If An Answer Works

Estimate to See if an Answer Works
Use estimation to verify the reasonableness of calculated results.
Mathematical Reasoning: estimation, reasonableness, problem solving The ability to use a reasonable guess to figure out mathematical problems and the ability to know when it’s appropriate to use a guess and when it’s appropriate to use a precise calculation.

Sample Problems


Is estimation reasonable?

Myles is practicing for a 100 -yard dash. He wants to know how much greater his trial time is than the race record time of 9.4 seconds.(Myles needs to know exactly how much faster he needs to go to beat the record, so estimation is not a good strategy.)


Is estimation reasonable?

Sheri is on duty as a lifeguard. It is 11:39 am. She wants to stop by the market on the way home. At what time shall she tell his friends she will arrive at the market? (Sheri does not need to arrive at the market at an exact time, so she can estimate the time.)


Estimate: 71 + 38 + 56 + 10 + 43 + 37 + 22 = ?


Use front-end estimation to add 53 + 98. (50 + 90= 140)


Use rounding to estimate 569 + 453. (570 + 450 = 1020)

Learning Tips


Teach children the phrasing that indicates an estimate is needed rather than an exact answer. Wording such as, “about how much.” Help children to ask, “What kind of estimation makes sense with this problem?”


Encourage the child to move from estimation to calculation, then back again to estimation. It’s important that students understand the concept behind the algorithm, so help them to talk through what makes sense before they solve a problem. The child should take time to look at their answers, after calculation, to see if they make sense. Children may need guidance in useful strategies for estimation, e.g., rounding numbers, creating a visual image, etc.


Front-end estimation refers to estimating a number by making the ones (or more) digit(s) a 0. Keep the two highest place value digits and turn the rest to 0’s. For example, 3765 would be 3700 and 8397 would be 8300.


Round each number and then add or subtract the rounded numbers. Rounding to the nearest ten, hundred, thousand, etc. is another way to estimate. Use rounding to determine whether you have enough money to buy what you want or to get a rough idea of the correct answer to a math problem.

Make sure that the child understands the difference between rounding and estimation. Rounding is a specific skill with rules that must be followed, whereas estimating is used to check the reasonableness of an answer either before or after a question is answered. There are no hard rules when estimating.


Take the learning further by asking the child which estimate is closer to the exact sum, rounding to the nearest ten or front-end estimation. Why does he/she think that is so? (front-end estimation will consistently fall behind). Is there a way to adjust estimation for 2-digit sums to make the process better?

Front-end estimation keeps falling behind the exact sum, while rounding is sometimes behind and sometimes ahead. Using this problem as an example: 71 + 38 + 56 + 10 + 43 + 37 + 22 = ?

--Add a 5 to the front-end estimate for every number in the sum, and use the Look for Tens strategy to add this to the front-end estimate. Rounding to the ones place makes a number estimate that is too large by up to 5 or too small by up to 4.  Front-end estimating makes an estimate that is never too large, but can be too small by up to 9. If you add 5 to every front-end estimate, the estimate is now too small by up to 4 or too large by up to 5--the new estimate is sometimes ahead and sometimes behind, just as it is for rounding.

Extra Help Problems


Estimate: 14 + 45 + 62 = (120)


Estimate: 86 + 23 + 62 = (170)


Estimate: 165 - 89 = (80)


About how many hours will it take to fly to another part of the United States? (Look at a map and choose a distant location.)


Your family plans to drive to a city about 400 miles away. You will be driving on a road where the average speed is 60 mph. About how long will the trip take? Will you talk about travel time in terms of minutes, hours, or days? Estimate how many times your family will need to stop to eat a meal during this trip. If you leave home by 9 AM, will you be able to reach your destination the same day?


Estimate the height of a chair. 4 feet, 10 feet or 10 miles? (4 feet)


What’s closest to the height of a gallon carton of milk? 1 yard, 12 inches, 3 inches or 90 inches? (12 inches)


Estimate the width of a CD. Is it 1 foot, 1 inch, 4 inches or 16 inches? (4 inches)


About how high is a car? 20 feet? 5 feet? 13 feet? Or 2 feet? (5 feet)


About how wide is your hand? 2 feet, 3 inches, 12 inches or 1 foot? (3 inches)


4:50 in the morning is about what time? (5 am)


12:05 in the afternoon is about what time? (noon or 12:00pm)


About how far do you walk to school, if you walk 2.2 miles? (about 2 miles)


Estimate: 895-704=(200)


Which is closest to the size across a basketball? 3 feet, 45 inches, 50 inches or 1 foot? (1 foot)


About how much does a cat weigh? 8 ounces, 8 pounds, 20 pounds or 2 ounces? (8 pounds)


A glass of milk is about 8 ounces, 2 ounces, 3 cups or ½ gallon? (8 ounces)


About how many lollipops will comfortably fit in the palm of your hand? 19, 58, 5, or 25? (5)


Jason wants to dig a hole in the backyard to China. Of course he’ll never get there, but how deep do you think he would have to dig to try? inches, yards or miles? (miles)


Bobby went to a friend’s house 1 street over. Did he walk miles, yards, feet or inches? (yards)


If you were to weigh yourself, would you be closer to 80 pounds or 80 kilograms? (pounds)


Estimate: 1,005 + 2,042 = (3,000)


Estimate: A baker cooked 22 loaves of bread and they had to each rise 2 hours. About how long did they all rise if she made each of them at a different time? (40 hours)


You’re going shopping and you need to buy a $5.53 T.V. dinner and a drink for $1.99. About how much do you need to bring to the store? ($8.00)


Your mom asks you to pick up $9.89 worth of groceries and $27 worth of dry cleaning on your way home. About how much money do you need to bring? ($40)


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