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Lei's Proportion problem

http://en.wikipedia.org/wiki/Rubens_apple

1. If apples cost $6,how much would 12 apple cost ?
Let x be the cost for 12 apples

It will cost $24 for 12 apples

http://chicolockersausage.com/2011/12/30/what-is-really-in-a-hot-dog/

2.You can buy 2 hotdogs for $3, how much would it be for 4 hotdogs?

Let x be the cost for 4 hotdogs

The cost for 4 hotdog is $6

Your problem :
You estimate that you can do 10 math problems in 30 mins. How long should it take you to do 20 math problems?

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