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Question:

A cup is 0.289 L. Approximately how many cups of coffee would it take to fill a 3.4 L coffee carafe?

A
10

Explaination

When estimating, we are trying to find a value that is close to the answer, but doesn’t have to be exact. We round each number and then solve, which is how we arrive at our estimate of the answer.

When estimating, we are trying to find a value that is close to the answer, but doesn’t have to be exact. We round each number and then solve, which is how we arrive at our estimate of the answer.

Rounding is when we make a number “simpler” but keep the value as close to the original number as possible. To round a number:

Determine which digit or place (value) is the number you want to “round to.” All numbers after this (to the right) will become zeroes after rounding.

Keep it the same number if the next number (the number to its right) is less than 5. Then, change all other numbers to the right to 0. For example, 132 rounded to the nearest ten would be 130. (132 is closer to 130 than it is to 140.)

Increase the number by 1 if the next number (the number to its right) is 5 or more. Then, change all other numbers to the right to 0. For example, 158 rounded to the nearest hundred would be 200. (158 is closer to 200 than it is to 100.)

In order to get close to the actual answer, round each number to the second largest place value.

For example:

325.13 → 330

25 → 30

5.7 → 6

5.78 → 5.8

For this problem:

0.289 → 0.3

3.4 → 3

Then we continue with the problem. In this case, we are dividing the carafe (3 liters) by the number of cups (0.3 liters):

3 ÷ 0.3 = 10 cups*

*If you do not have access to a calculator for this question, you may solve the final step like this:

Convert the decimal into a fraction (it reads, “three tenths” so you can simply write that out at a fraction):

3 ÷ 3/10 =

When dividing a whole number by a fraction, first convert the whole number into a fraction by making the whole number the numerator and 1 the denominator.

3/1 ÷ 3/10 =

Then, because you are dividing by a fraction, flip the second fraction and multiply.

3/1 • 10/3 =

You can cross reduce the 3’s by dividing both by 3. (Or you can multiply the two fractions as they are and reduce the answer.)

3÷3/1 • 10/3÷3 = 1/1 • 10/1 = 10

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