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?  Choices.
A) 2 B) 3 C) 5 D) 7 E) 9
Explanation :   (8+x)/(21+x) = (13+x)/(31+x)   Then, (8 + x)(31 + x) = (13 + x)(21 + x)   or39x + 248 = 34x + 273 or 5x=25 or x = 5.   Answer : 5
<result>C</result>
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?