Primary 4 GEP

by Joyce sim
(Singapore)

Average of a number of files is $4. After buying another file which cost $16, the average increases to $7. Find the number of files at the beginning.



Here's My Answer
by Zach
(Singapore)



Step 1: This question involves the Total Vs Individual Difference Concept. First, we draw 1 box to represent the old average of $4. Then we draw an arrow to the right to show that this box is repeated by an unknown number of times (label "?" at the end of the arrow).

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Mar 18, 2022
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starstarstarstarstar 		alternative answer
by: VTF

avg=4 4 4 4 4 4...4
avg=7 4 4 4 4 4...4 16

extra file to split into 4+12
avg=7 4 4 4 4 4...4 4 12

the average difference=7-4=3
the excess=12
# of file=12/3=4
at the beginning is 4-1=3!
Oct 11, 2010
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starstarstarstarstar 		Sorry for the mistake.
by: Zach

Yes Joyce, you are right. So sorry for the error. It should be $9 not 9 files. I'll amend it later. Thanks for pointing that out. Hope the answer was of help to you and your kids.
Oct 11, 2010
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starstarstarstarstar 		A query
by: Joyce

Zach, thank you for the prompt response and helping me to answer this question using model method. Just one question. In step 3, you mentioned that the total between the 1st and 2nd scenario is 9 files. Shouldn't it be $9?

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