Estimating products
Free lesson plan from HomeschoolMath.net

Estimate first and use that to check the result.  8 × 67 = ?
 

67 can be rounded up to 70, or 67 ≈ 70.  Therefore 8  × 67 ≈ 8  × 70 = 560.

5    67 ×  8 356 5    67 ×  8 536 Oops!  356 is  far from the  estimation 560. There must be  an error. Can  you find it? This result  sounds right, because it is fairly close to  560.

Before we start using estimation as a checking method, let's review the rounding rules.

When rounding to nearest ten, 

• If the last digit is  __, __, __, or __, round down to the previous ten.
• If the last digit is __, __, __, __, or __, round up to the next ten.

The symbol  ≈  is used to indicate rounding, and is read as "is approximately" or "is about".