Estimating products and quotients of fractions 

Just like the previous lesson, we are going to discuss two strategies.   

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Strategy 1 

Determining the upper and the lower bounds of the product and quotient  

Example: 

1.)In the fractions:  ,the upper bound of 5 x 3 is obtained by rounding both fractions off to the next highest whole number, while the lower bound of 4 x 2 is obtained by multiplying only the whole numbers.  This means that the answer lies between 8 and 15. 

2.)In the fraction :  ,  we see that by rounding off to the nearest whole and dividing the two wholes,,  are not the upper and lower bounds expected because both of these answers are smaller than the actual quotient.  Both of these provide a reasonable estimate of the quotient. 

3.)  In the computation , we find that  and  , are the upper and lower bounds respectively.  It is advisable to only round the divisor up, or down. 

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Strategy 2 

Get a rough estimate by using compatible numbers   

Example: 

Peter spends a quarter of his monthly pocket money of R 275 on sweets.. How much money does he spend monthly on sweets? 

Suggested estimation:  

      of R275                                                  or                           x (200 +80)= 

     x 300                                                                                   =  50 + 20

=                                                                                            =R 70

= R75 

These 2 examples are good examples for the given situation.The last example is the closest estimation of his monthly expenditure on sweets of R 68,75.

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Previous estimation strategies:

Lesson 1

Lesson 2