Concept

To estimate the result of multiplication (product), round the numbers to some close numbers that you can easily multiply mentally. 

Rule

1. Determine a compatible whole number that is a multiple of the denominator of the fraction.

2. Calculate the product

Example

Which is the best estimate for 18 × 42?

Use compatible numbers to estimate.

Solution

Multiplying by one-eighth is the same as dividing by 8.

Determine a whole number that is compatible to the denominator of the fraction.
A compatible whole number would be a multiple of 8 that is close to 42. 
40 is the closest compatible number.

Write 40 as a fraction. 401

Multiply to find an estimate product.
18 × 401 = 408

Simplify the fraction.
408=5

Pre-requisite Skills

Round Fractions (5.7.5)

Find the mode (5.8.6)