Estimate the volume of a rectangular pool that has a depth of 2.49 m, a length of 20.1 m and a width of 9.8 m. The volume of a rectangular pool can be calculated by multiplying length ✕ width ✕ depth.

Question

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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:
2.49 → 2.5
20.1 → 20
9.8 → 10
Now solve:
Volume = L ✕ W ✕ D
Volume = 20cm ✕ 10cm ✕ 2.5cm
Volume = 500

Estimate the Volume of a Rectangular Pool That Has a Depth