Estimation is like making a smart guess about numbers. Instead of finding the exact answer, we find a number that is close enough. This helps us know if our exact answer makes sense.
Let’s learn how to estimate when adding two numbers. We look at each number and find the closest ten. For example, if we have 31 plus 18:
Now, we add 30 and 20 to get an estimated sum of 50. The exact sum of 31 plus 18 is 49. Since 49 is close to 50, our estimate is reasonable.
Let’s try another one: 36 plus 52.
The estimate is 90 because 40 plus 50 equals 90. The exact answer is 88, which is close to 90, so our estimate is reasonable.
Now, let’s estimate when subtracting two numbers. We find the closest ten for each number. For example, 89 minus 65:
The estimated difference is 20 because 90 minus 70 equals 20. The exact difference is 24, which is close to 20, so our estimate is reasonable.
Let’s try another example: 78 minus 53.
The estimate is 30 because 80 minus 50 is 30. The exact answer is 25, which is close to 30, so our estimate is reasonable.
When we estimate with money, we figure out about how much we need to buy things. This helps us know if we have enough money.
For example, Joe has 50 cents and wants to buy a kite and a yo-yo. Let’s see if he has enough money:
Our estimate is 70 cents because 30 plus 40 equals 70. Joe has 50 cents, so he doesn’t have enough money.
Now, Jackie has 95 cents and wants to buy two kites. Let’s check:
So, 40 plus 40 is 80. Our estimate is 80 cents. Jackie has 95 cents, so she has enough money to buy two kites.
Estimation is a handy tool to quickly check if our answers make sense and to help us with everyday decisions!
Estimation Treasure Hunt: Go on a treasure hunt around your home or classroom to find items with price tags or numbers. Try to estimate the total cost or sum of these items by rounding each number to the nearest ten. For example, if you find a book priced at 23 cents and a toy priced at 47 cents, round them to 20 and 50, respectively, and estimate the total cost. Share your findings with a friend or family member and see if they agree with your estimates!
Estimation Art Project: Create an art project using different colored paper or materials. Assign a number to each color (e.g., red = 12, blue = 27, yellow = 35). Use these numbers to estimate the total number of pieces you will use in your artwork. Round each number to the nearest ten before adding them up. Once your artwork is complete, count the exact number of pieces used and compare it to your estimate. How close were you?
Estimation Story Time: Write a short story about a character who uses estimation to solve a problem. For example, your character might be planning a party and needs to estimate how many balloons and cupcakes to buy. Use examples from the lesson to show how they round numbers and make smart guesses. Share your story with your classmates or family and discuss how estimation helped your character.
Here’s a sanitized version of the provided transcript:
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**Estimating Addition, Subtraction, and Money**
When we use estimation, we are finding an approximate or close sum or difference to an addition or subtraction problem. We use estimation when the problem asks us to find out about how many or about how much. In these problems, we can estimate instead of finding the exact answer. Sometimes it is helpful to estimate the sum or difference first so that we have an idea of what the exact answer will be. This is also helpful to check if our exact answer is reasonable.
**Estimating Addition**
To estimate a two-digit addition problem, we find the closest ten for each number in the problem. For example, for 31 plus 18:
– 31 is closest to 30 because 1 is less than 5, so we round down.
– 18 is closest to 20 because 8 is greater than 5, so we round up.
Now we can easily use mental math to find the estimated sum. The estimated sum is 50 because 30 plus 20 equals 50.
Now we can solve for the exact sum: 31 plus 18 is 49. To check if our exact sum is reasonable, it needs to be no more than five away from the estimate or no less than five away from the estimate. Our exact sum is only one less than the estimated sum, so our sum is reasonable.
Let’s try another one: 36 plus 52.
– 36 is closest to 40 because 6 is greater than 5, so we round up.
– 52 is closest to 50 because 2 is less than 5, so we round down.
The estimate is 90 because 40 plus 50 equals 90. The exact answer is 88 because 36 plus 52 is 88. Our exact answer is two less than the estimate, so our answer is reasonable.
**Estimating Subtraction**
To estimate a two-digit subtraction problem, we find the closest ten for each number in the problem. For example, for 89 minus 65:
– 89 is closer to 90 because 9 is greater than 5, so we round up.
– 65 is closer to 70 because 5 is equal to 5, so we round up.
Now we can easily use mental math to find the estimated difference. The estimated difference is 20 because 90 minus 70 equals 20.
Now we can find the exact difference: 89 minus 65 is 24. To check if our exact difference is reasonable, it needs to be no more than 5 away from the estimate or no less than 5 away from the estimate. Our exact difference is 4 more than the estimated difference, so our difference is reasonable.
Let’s try another example: 78 minus 53.
– 78 is closest to 80.
– 53 is closest to 50.
The estimate is 30 because 80 minus 50 is 30. The exact answer is 25 because 78 minus 53 is 25. Since our exact answer is 5 less than the estimate, our answer is reasonable.
**Estimating Money**
When we estimate with money, we are usually trying to figure out the approximate amount needed to purchase certain items. To determine if we will have enough money to buy certain items, we use estimating and addition.
For example, Joe has 50 cents and would like to buy a kite and a yo-yo. We need to figure out if Joe will have enough money to buy these two items, so we will estimate about how much money he will need.
The yo-yo is 27 cents, so we round up to 30, and the kite is 42 cents, so we round down to 40. Our estimate is 70 cents because 30 plus 40 equals 70. Joe has 50 cents and needs about 70, so Joe does not have enough money to buy these items.
Now, Jackie has 95 cents and wants to buy two kites. Does she have enough money to buy two kites? The kite is 42 cents, which rounds to 40. Therefore, 40 plus 40 is 80. Our estimate is 80 cents. Jackie has 95 cents and needs about 80 cents, so Jackie does have enough money to buy two kites.
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