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Tom had a total of 120 red and blue marbles. 30% of them were red marbles. His father gave him some more red marbles, after which the percentage of red marbles increased to 40%. How many red marbles did Tom's father give him?

 Aug 10, 2015

Best Answer 

 #1
avatar+28130 
+5

30% of 120 is 36, so Tom originally had 36 red marbles.  If his father gave him n red marbles then

 

36+n = (120 + n)*40/100   Multiply through by 10 and simplify:

 

360 + 10n = 480 + 4n   Subtract 4n from both sides

 

360 + 6n = 480   Subtract 360 from both sides

 

6n = 120  Divide both sides by 6

 

n = 20

 

Tom's father gave him 20 red marbles.

.

 Aug 10, 2015
 #1
avatar+28130 
+5
Best Answer

30% of 120 is 36, so Tom originally had 36 red marbles.  If his father gave him n red marbles then

 

36+n = (120 + n)*40/100   Multiply through by 10 and simplify:

 

360 + 10n = 480 + 4n   Subtract 4n from both sides

 

360 + 6n = 480   Subtract 360 from both sides

 

6n = 120  Divide both sides by 6

 

n = 20

 

Tom's father gave him 20 red marbles.

.

Alan Aug 10, 2015

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