Question:What number should be added to each of the numbers `8, 21, 13` and `31` so that the resulting numbers, in this order form a proportion ? 

A `2` 
B `3` 
C `5` 
D `7` 

+ Answer
C 

+ Explanation
Let `(8 + x)/(21 + x) = (13 + x)/(31 + x)`.

Then, ` (8 + x)(31 + x) = (13 + x)(21 + x)`

or `39x + 248 = 34x + 273`

or `5x = 25`

or ` x = 5`.

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Md. Habib Ullah

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