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Mrs Lao bought 420 blue pens and 160 red pens. She sold some of the red pens. After that, 7/9 of the pens left were blue pens. How many red pens did Mrs Lao sell?

 Aug 5, 2015

Best Answer 

 #1
avatar+107355 
+5

Let's call the number of red pens sold = S   so...we have

 

The number left x (7/9)  = 420...or, mathematically.....

 

[420 + 160 - S] (7/9) = 420     multiply both sides by (9/7)

 

[420 + 160 - S]  = 540

 

580 - S  = 540       rearrange

 

580 - 540 =  S  = 40   red pens sold

 

 

 Aug 5, 2015
 #1
avatar+107355 
+5
Best Answer

Let's call the number of red pens sold = S   so...we have

 

The number left x (7/9)  = 420...or, mathematically.....

 

[420 + 160 - S] (7/9) = 420     multiply both sides by (9/7)

 

[420 + 160 - S]  = 540

 

580 - S  = 540       rearrange

 

580 - 540 =  S  = 40   red pens sold

 

 

CPhill Aug 5, 2015

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