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You should read the posting on Sarin learns Average Number, Percentage and Charting in school (Math Concept) to understand the concept of Percentage.
Upper primary school mathematics question UPQ148
Sarin’s mother is keeping track of the household expenses. She need to make sure the family’s expenditure is within their means. The line chart below shows their family’s expenditure over five weeks. When was there a 20% decrease in their family’s expenditure?
Solution:
The percentage of decrease is the percentage of decrease in expenditures of two consecutive weeks to the expenditure of the first week of the two weeks.
Base on the linechart, we can tabulate a table with the weekly decreases and compute the percentage of decreases.
Example:
The different in expenditure from 1^{st} week to 2^{nd} week = 90 – 80 = $10
The percentage decrease = 10 ÷ 90 × 100% = 11.1%

Expenditure ($) 
Decrease ($) 
Decrease (%) 
1^{st} week 
90 


2^{nd} week 
80 
10 
11.1 
3^{rd} week 
50 
30 
37.5 
4^{th} week 
40 
10 
20.0 
5^{th} week 
20 
20 
50.0 
From the table, the family expenditure decrease by 20% from 3^{rd} week to 4^{th} week.