Question

How do you estimate `1087times21` by rounding?

Answer 1

`~~21800`

Explanation:

First, round-off the given numbers.
Note that in general, the degree of rounding will affect how close the estimated product will be to the actual product. The estimated value when we round-off to the nearest tens will usually be closer to the actual value if we round-off to the nearest hundreds or even thousands. However, do also take note that the higher the degree of rounding off, the easier it is compute the estimated value. At some point, we may be able to mentally compute the estimated value

Let's round off 1087 and 21 to the nearest tens

`1087 => 1090`
`21 => 20`

`1087 * 21 ~~ 1090 * 20 ~~ 21800`

If we round off 1087 to the nearest hundreds and 21 still to the nearest tens, we have

`1087 => 1100`
`21 => 20`

`1087 * 21 ~~ 1100 * 20 ~~ 22000`

If we round off 1087 to the nearest thousand and 21 still to the nearest tens, we have

`1087 => 1000`
`21 => 20`

`1087 * 21 ~~ 1000 * 20 ~~ 20000`