At the garden centre

At the garden centre

Pencil and paper
Overview
Using this Resource
Connecting to the Curriculum
Marking Student Responses
Working with Students
Further Resources
This task is about rounding and multiplying prices.
 
Jackie was shopping at a garden centre. She estimated the cost of her purchases by:
  • rounding the cost of a single plant to the nearest ten dollars, and
  • multiplying it by the number of plants she wanted.
Write down Jackie's estimate for each group of items.
 
a) 25 roses costing $12.50 each. $__________
 
b) 12 trees costing $23.95 each. $__________
 
c) Five sets of terracotta pots costing $74.75 per set. $__________
 
d)
Why is Jackie's method of estimating the total bill not very accurate?
 
 
 
 
 
e)
How could she get a more accurate estimate while still using numbers that are easy to multiply in her head?
 
 
 
 
Task administration: 
This task is completed with pencil and paper only.
Copyright: 
Image source: Wikipedia, 2009
Level:
5
Curriculum info: 
Maths, Number and Algebra, Number Strategies
Key Competencies: 
Using language, symbols, and texts
Keywords: 
estimation, rounding
Description of task: 
Students use rounding to estimate totals then discuss this as a method of estimation.
Curriculum Links: 
Key competencies
This resource involves explaining how they made an estimate, and how to make a more accurate estimate. This relates to the Key Competency: Using language, symbols and text.
For more information see http://nzcurriculum.tki.org.nz/Key-competencies
 
Learning Progression Frameworks
This resource can provide evidence of learning associated with
Multiplicative thinking, sets 8-9
within the Mathematics Learning Progressions Frameworks.
Read more about the Learning Progressions Frameworks.
Answers/responses: 
   

Y10 (11/1999)
a) 250 moderate
b) 240 moderate
c) 350 moderate
d)
Any 1 of:
  • It isn't very accurate because all the numbers have been rounded down.
  • The difference between the actual price and the  rounded price is nearly $5 for some items.
  • Sensible alternatives.
 difficult
e)
Any 1 of:
  • Round to the nearest five dollars.
  • Allow for the fact that she had rounded down  all the numbers in her final sum, e.g., round one  of the tems up to the next ten.
  • Sensible alternatives.
 difficult
Diagnostic and formative information: 
  Common error Likely calculation Likely reason
a)
b)
c)		 312.50
287.40
373.75		 25 × 12.50
12 × 23.95
5 × 74.75		 Exact answer.
a)
b)
c)		 310
290
370		    Rounds exact answer to nearest $10.
a)
b)
c)		 325
288
375		 25 × 13
12 × 24
5 × 75		 Rounds to nearest $1.